SendStandbyStatusUpdate should be called periodically

In the "main" ReplicationManager loop, there is a chance that LR processing is blocked while replicating from one NATS stream (snapshot) to another NATS stream (main) is running.

If we don't send the SendStandbyStatusUpdate message to PostgreSQL in time, the replication stream connection will be closed.

Proposal:

Implement SendStandbyStatusUpdate calls with the latest LSN to a separate goroutine. We need to prevent concurrent access to the underlying connection so locks might be needed.