Synchronize maildir syncback (#137)

It is possible for maildir sync back to occur during polling. To prevent this,
syncback now locks the source before writing back labels.

poll_lock is now a monitor instead of a mutex, so reentrant locking is
possible. Sources can be locked directly using Source#try_lock, Source#unlock
and Source#synchronize.

Moved some syncback logic from Message to Location. The configuration check
for syncback is now also checked there, so we can add source-specific syncback
per source (see #141).
Merge pull request #137 from ericweikl/fix-137-poll

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