More fine-grained statuses for MicroCluster

MicroCluster requires **all** nodes in a cluster to have the same schema/API version and if update will be triggered when one of the nodes is offline, the update will never finish.

I was thinking if we can prevent such update from happening by adding some guard rails (deb `preinst` or snap `pre-refresh`) before doing an update? But it seems to be tricky.

1. `member1` checks cluster status before upgrade, if **all** `online`, then it proceeds
```
+---------+-------------+
| NAME | STATUS |
+---------+-------------+
| member1 | ONLINE |
+---------+-------------+
| member2 | ONLINE |
+---------+-------------+
| member3 | ONLINE |
+---------+-------------+
```

2. `member1` is updated

cluster status from `member1`
```
+---------+---------------+
| NAME | STATUS |
+---------+---------------+
| member1 | UPGRADING |
+---------+---------------+
| member2 | NEEDS UPGRADE |
+---------+---------------+
| member3 | NEEDS UPGRADE |
+---------+---------------+
```

cluster status from `member2`
```
+---------+-------------+
| NAME | STATUS |
+---------+-------------+
| member1 | UNREACHABLE |
+---------+-------------+
| member2 | ONLINE |
+---------+-------------+
| member3 | ONLINE |
+---------+-------------+
```

Any reason the heartbeat endpoint couldn't be made to return a status indicating the `member1` is online but waiting for upgrade instead of unreachable?

P.S. I am using this as a current workaround:
```go
var healthCmd = rest.Endpoint{
Path: "health",
AllowedBeforeInit: true,
Post: rest.EndpointAction{Handler: cmdPost, AllowUntrusted: true},
}

func cmdPost(state state.State, r *http.Request) response.Response {
return response.SyncResponse(true, state.Database().Status())
}
```

Start by tracing the heartbeat and health endpoints, including the cmdPost handler and state.Database().Status() shown in the workaround. Determine how a node that is online but waiting for upgrade is currently reported and define the additional status needed during a rolling upgrade. Done means cluster status reports that state consistently instead of marking the upgraded member unreachable.

Written by the indexing model from the issue text.