Looks like eagerly synchronizing tags is easy and works fine, though I need to beef up the tests and put some transactions around things before I'm satisfied.

I added a "notmuch_database_remove_message_get" to the public API that's just like "notmuch_database_remove_message" except that it also returns a notmuch_message_t if other instances of the message still exist.  It feels clunky to have two almost identical variants of this function.  Is this the preferred way to change the public API?  Or should I just add the argument to the existing function and fix the other three calls to it?