R1:326d5bf8004e

Detect replicating masters and fatal (also, warn on nonreplicating replicas)

Summary:
Ref T10759. Check master/replica status during startup.

After D16903, this also means that we check this status after a database comes back online after being unreachable.

If a master is replicating, fatal (since this can do a million kinds of bad things).

If a replica is not replicating, warn (this just means the replica is behind so some data is at risk).

Also: if your masters were actually configured properly…