Proposal: Add configurable Server Guild attribute to enable the coordinator to make replication decisions that spread replicants across operator defined guilds

***Note***: I use the term "Guild" throughout this proposal. It is an arbitrary name that I chose for the Historical grouping construct that I am proposing. A Guild would be a group of Historical servers. All servers who do not specify a Guild in runtime.properties would be assigned to a default Guild.

### Motivation

As of now, the only guarantee for segment loading is that no two replicants will be loaded onto the same Historical Server. You can also use tiering to load a specific number of replicants onto specified tiers for a datasource via load rules. As an operator of a large cluster, I want to maximize segment availability and optimize maintainability of my cluster. To achieve goal number one, I want my cluster to do its best to load replicants to groups of servers that have the lowest likelihood of going down simultaneously causing loss of segment availability. To achieve goal number two, I want my cluster to do its best to load replicants to groups of servers that allow me to perform maintenance (such as service restarts, OS patching) on groups of servers at one time without worrying about losing segment availability while doing so. Both of these goals can be achieved by grouping historical servers and making the coordination algorithm aware of the groups. The coordination algorithm can then use the groups as input into its decision making on where to load segments on the cluster, making best effort to load replicants across 2+ groups. If this is done, I can, as an operator, set up my groupings to meet my goals.

### Proposed changes

I have a POC going in my fork of Druid. The code can be viewed [here](https://github.com/capistrant/incubator-druid/tree/guild-replication-poc2).

**Configs**
runtime.properties:
* `druid.server.guild` The STRING guild name assigned to a server. Default value applied if not specified.
* `druid.coordinator.guildReplication.on` The BOOLEAN flag telling the coordinator if it should use server guilds as a part of its coordination logic. Default value is false.

coordinator dynamic config:
* `guildReplicaitonMaxPercentOfSegmentsToMove` What % of segments moved during BalanceSegments duty should be dedicated to moving segments that are not meeting guild replication threshold of 2? This is applied against what is left over after moving segments off of decommissioning servers, if there are any.

`SegmentReplicantLookup`

The coordinator currently builds this object each time the `HistoricalManagementDuties` run. Prior to this proposal, the object would contain two main data structures:

`Table segmentsInCluster` - replicant count for each segment by tier for segments served by the cluster.
`Table loadingSegments` - replicant count for each segment by tier for segments being loaded by the cluster.

My proposal adds a third data structure that is specific to guild replication:

`Table historicalGuildDistribution` - replicant count for each segment by guild.

This new structure is only created if guild replication is enabled. It is not worth the resources if we are not going to use it! The structure is used for quick lookup to get guild replication state for a given segment. It will be used by coordinator duties when making decisions on loading/moving/dropping segments.

`LoadRule`

When Assigning replicas, use the changes in SegmentReplicantLookup in order to split `ServerHolders` into groups of servers who are on a guild that is serving a replicant of the segment and servers who are on a guild that is not serving a replicant of the segment. If possible `LoadRule` will load replicants to the best scored server(s) from the guild(s) not serving a replicant. However, we always fallback to just loading the specified number of replicants even if replication across guilds cannot be achieved.

When picking replicants to drop, we will also split the `ServerHolders` in the same way. The segments will be dropped from decommissioning servers first, then from servers on guilds with > 1 replicant of the segment, then lastly from the remaining servers serving a replicant.

`BalancerStrategy`

Add a method to the interface. `pickSegmentToMove(List serverHolders, Set broadcastDataSources, DruidCoordinatorRuntimeParams params);`

This new method is added so we have the `SegmentReplicantLookup` information needed to pick a segment who is not replicated across > 1 guild when balancing the cluster. It is needed because we introduce the dynamic config and associated balancing phase in BalanceSegments that prioritizes the balancing of segments who are not properly replicated across guilds. We need to only pick segments that meet the requirements. The existing `pickSegmentToMove` does not suffice.

RandomBalancerStrategy and CostBalancerStrategy add implementations.

`ReservoirSegmentSampler`

Adds a method for getting a `SegmentHolder` that is violator of the goal of being on > 1 guilds. This method needs to have access to the `SegmentReplicantLookup` in order to quickly look up replication state of segments it is possibly selecting. It returns the first violator that it finds or null if none is found.

`BalanceSegments`

The coordinator balancing duty gets a couple of changes. The first change is to the generic balancing that exists today. If guild replication is enabled, then we will perform the split of `ServerHolders` based on their guild replication status when looking for a server to move a segment to. Just as in `LoadRule` we will do our best to make the move to a server that improves or maintains the number of guilds that hold a replicant.

We also add a new phase of balancing segments. There is a dedicated move for segments off of decommissioning servers. An operator can also choose to add a dedicated move for segments that are not replicated on > 1 guild. They do this by editing the dynamic coordinator config that this proposal adds. This results in the coordinator moving a certain number of segments that are violating guild replication rules. It is an optional way for an operator to speed up the balancing of segments across guilds.

### Rationale

There has been some feedback that this functionality should be folded into the tier construct that already exists. I tend to disagree with that idea for multiple reasons.
* Tiers have been around for a long time and have become cornerstones of operator's deployments. It may not be easy or straightforward to re-purpose tiers to also work as generic replication groups. I fear we'd be creating a single monster that is hard to understand vs having two separate concepts split out into their own things (guilds vs tiers)
* As written, tiers are explicit. I load N replicants onto Tier A, M onto Tier B, etc. Whereas guild replication is a generic best effort goal. I try to load the aggregate replicants across all tiers onto 2 or more guilds. We can't change tiers to be generic with loading to go across 2 or more tiers, because operators expect their load rules to be followed to a T. So we end up with competing goals that do not play nicely together. This ties back to my point that trying to adapt tiering to meet so many goals could create a confusing beast.
* My motivating goal for uptime and maintainability may require a separate grouping outside of my existing tiering. Perhaps servers for my different tiers are mixed and matched within logical groupings that I would need to separate out in order to achieve my motivating goals. If my grouping for uptime and maintainability is in regards to physical racks. I could have my slow and fast servers racked on the same set of racks with a few of each form-factor on any given rack. This means I cannot achieve my same tiers for my speed tiering as well as my replication goals. I would need to further split each rack into tiers, which once again is getting complex and probably messing up performance by adding way too much isolation vs shared resources.

So now that adapting tiering is ruled out as far as I am concerned, what else could we do? I can't think of other solutions for the configurations that are as generic as my simple guild name assignment and boolean for the coordinator on whether or not to follow guild replication logic paths. But what about the implementation details using these configurations? It could certainly be argued that the cost of the new `SegmentReplicantLookup` data structure is too steep for large clusters. However, I fail to come up with a better solution in my head. When we are choosing servers to load/drop segments from, we need a quick way to look up details on where that segment is loaded. It seems logical that this facility would require this large structure in the replicant lookup.

With all this being said, I am open to improvements to my proposal. At the end of the day my motivating factors just need to be achieved. If there is a better way to achieve the simple goals required, I'm all for it.

### Operational impact

This section should describe how the proposed changes will impact the operation of existing clusters. It should answer questions such as:

**- Is anything going to be deprecated or removed by this change? How will we phase out old behavior?**

Nothing is deprecated or removed.

**- Is there a migration path that cluster operators need to be aware of?**

To go from not using guild replication, to using guild replication, there will be a simple migration path. Operators will need to assign guilds to their historical servers and restart them. They will then have to flip the coordinator config to turn on guild replication. They may choose to use the coordinator dynamic config to speed up balancing based on guild replication if they so choose.

**- Will there be any effect on the ability to do a rolling upgrade, or to do a rolling _downgrade_ if an operator wants to switch back to a previous version?**

Upgrading to the first version of druid that has guild replication in it will not require any special work. Upgrading to the default configs will keep guild replication off. If they choose to use guild replication, they can perform the migration steps after the upgrade. If they choose not to use guild replication, no action is needed! Downgrading would require a pre-step by an operator if they have already turned guild replication on. They would need to turn guild replication off on the coordinator by updating their config and restarting it. That way there will not be any issues with split versions across coordinator/historical during downgrade.

**- Other**

This change as proposed results in additional resource utilization by the Coordinator. Extra data structures are created in `SegmentReplicantLookup` that help make replicant loading decisions based on guild distribution of segments. This may require attention from operators as far as configuring their runtime environment for the coordinator.

The changed as proposed introduces a major change to the decision making process by the Coordinator. Enabling guild aware replication may result in replicant loading decisions that the coordinator would have previously considered sub-optimal. Loading replicants to different guilds to gain better uptime and maintainability standards could result in segment distribution skew that negatively impacts performance. It is a trade-off that an operator will need to carefully consider.

### TODO (for completeness of this proposal)

* ~~I believe I still need to add logic for what to do when there is only 1 total replicant expected for a segment across all tiers. If this is true, then there really is not a reason to make guild related loading/balancing decisions for it since there is only ever 1 replicant.~~ This one is tough to deal with. We'd have to dive into load rules for each segment we are evaluating for moving to take this into account during BalanceSegments
* Evaluate need for integration tests
* Evaluate possibility for simulation tool. Could we take a cluster snapshot and produce a simulated output of the cluster state once it is balanced for guild replication?

### Test plan (optional)
### Future work (optional)

Start by reviewing the linked guild-replication POC and the mentioned SegmentReplicantLookup, LoadRule, BalancerStrategy, ReservoirSegmentSampler, and BalanceSegments entry points. Trace how guild configuration and replication distribution would affect loading, dropping, and balancing decisions. Done means the proposed configurations and best-effort cross-guild replication behavior are implemented without changing default behavior when disabled.

Written by the indexing model from the issue text.