Maildir-syncback: Synchronize with poll_lock (possibly in a non-blocking way/or a separate thread) in all operations that write a new message or changes a message

Some of the operations in maildir-syncback are not locking on @poll_lock, risking a running poll operation while or during an operation that writes a message.

@ericweikl: Putting you as responsible since you're our maildir-syncback guy.