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.
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