Fast-forward replication through transitive checkpoint analysis

## Summary

I'd like to be able to choose the starting sequence for a replication between a given source and target using more information than just the replication history between those two databases. Specifically, I'd like to be able to use other replication checkpoint histories to discover transitive relationships that could be used to accelerate the first replication between CouchDB databases that share a common peer.

## Desired Behaviour

It might be simplest to provide an example. Consider a system where you have a pair of cloud sites (call them `us-east` and `us-west`) and a series of edge locations (e.g. `store1`):

* `us-east` and `us-west` are replicating with each other
* `store1` is pulling data from `us-east`
* `us-east` experiences an outage, so we respond by initiating `us-west` -> `store1`

In the current version of CouchDB, the `us-west` -> `store1` replication will start from 0 because those peers have no replication history between them. Going forward, it would be useful for us to recognize that `us-west` -> `us-east` has a history, and `us-east` -> `store1` has a history, so we can fast-forward `us-west` -> `store1` by analyzing the pair of those checkpoint histories to discover the maximum sequence on `us-west` guaranteed to have been observed on `store1` (by way of `us-east`).

## Possible Solution

I believe we actually already employ this transitive analysis for fast-forwarding internal replications between shard copies in a cluster, so we may be able to refactor some of that code to apply it more generally.

I'm not sure if we track the _target_ sequence in the current external replication checkpoint schema. That's essential for this analysis to work.

There's nothing fundamental that limits the analysis to first-order transitive relationships. One could build out an entire graph. I'm not sure the extra complexity that would bring is worth it in a first pass.

## Additional context

Proposing this enhancement after chatting with a user who is planning this kind of deployment and would benefit from the enhancement.

Start by examining the existing transitive analysis used for internal replications between shard copies, then inspect the external replication checkpoint schema to determine whether it tracks the target sequence. Done means external replications can use checkpoint histories across a shared peer to choose a safe fast-forward starting sequence.

Written by the indexing model from the issue text.