MM-50427: Make MM survive DB replica outage (#22888)

We monitor the health of DB replicas, and on a fatal error,
take them out of the pool.

On a separate goroutine, we keep pinging the unhealthy replicas,
and on getting a good response back, we add them back to the pool.

https://mattermost.atlassian.net/browse/MM-50427

```release-note
Mattermost is now resilient against DB replica outages and will
dynamically choose a replica if it's alive.

Also added a config parameter ReplicaMonitorIntervalSeconds
whose default value is 5. This controls how frequently unhealthy
replicas will be monitored for liveness check.
```

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/store/store.go

@@ -72,10 +72,7 @@ type Store interface {
 	// GetInternalMasterDB allows access to the raw master DB
 	// handle for the multi-product architecture.
 	GetInternalMasterDB() *sql.DB
-	// GetInternalReplicaDBs allows access to the raw replica DB
-	// handles for the multi-product architecture.
 	GetInternalReplicaDB() *sql.DB
-	GetInternalReplicaDBs() []*sql.DB
 	TotalMasterDbConnections() int
 	TotalReadDbConnections() int
 	TotalSearchDbConnections() int