nmhive.py: Commit tag changes to nmbug

There's a bit of a race here: we need to close the database before
committing so that the change has been flushed to disk for nmbug to
pick up, but that means we're not committing while protected by the
read/write lock.  Ideally, we could flush the database to disk, run
the nmbug commit, and then close the database to release the lock.
Unfortunately there is no "commit changes to the datatabase without
closing" command exposed by notmuch, but maybe I can talk folks into
adding one ;).  On the other hand, the race is against parallel
request that aquires the lock after we release it *and* gets their
commit in before ours, which seems unlikely.  Still, it would be
better if we had a known-safe option here.