lib: Only sync modified message documents

Previously, we updated the database copy of a message on every call to
_notmuch_message_sync, even if nothing had changed.  In particular,
this always happens on a thaw, so a freeze/thaw pair with no
modifications between still caused a database update.

We only modify message documents in a handful of places, so keep track
of whether the document has been modified and only sync it when
necessary.  This will be particularly important when we add message
revision tracking.