[WIP2 01/12] lib: Only sync modified message documents

From: Austin Clements 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.

[WIP2 01/12] lib: Only sync modified message documents

From: Austin Clements amdra...@mit.edu 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