[PATCH 1/6] lib: Only sync modified message documents

[David Bremner]

David Bremner <david at tethera.net> writes:

> From: Austin Clements <amdragon at 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 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.

This patch makes some sense without the rest of lastmod series, so I
merged it early to get some testing.

d