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/boards/services/store/sqlstore/migrate.go

@@ -70,7 +70,10 @@ func (s *SQLStore) getMigrationConnection() (*sql.DB, error) {
 	}
 	*settings.DriverName = s.dbType
 
-	db := sqlstore.SetupConnection("master", connectionString, &settings)
+	db, err := sqlstore.SetupConnection("master", connectionString, &settings, sqlstore.DBPingAttempts)
+	if err != nil {
+		return nil, err
+	}
 
 	return db, nil
 }