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/storetest/settings.go

@@ -261,6 +261,7 @@ func MakeSqlSettings(driver string, withReplica bool) *model.SqlSettings {
 	}
 
 	log("Created temporary " + driver + " database " + dbName)
+	settings.ReplicaMonitorIntervalSeconds = model.NewInt(5)
 
 	return settings
 }