The function notmuch_exit_if_unmatched_db_uuid is split from
notmuch_process_shared_options because it needs an open notmuch
database.
There are two exceptional cases in uuid handling.
1) notmuch config and notmuch setup don't currently open the database,
so it doesn't make sense to check the UUID.
2) notmuch compact opens the database inside the library, so we either
need to open the database just to check uuid, or change the API.
In the short term we need a way to get lastmod information e.g. for
the test suite. In the long term we probably want to add lastmod
information to at least the structured output for several other
clients (e.g. show, search).
This exposes the committed database revision to library users along
with a UUID that can be used to detect when revision numbers are no
longer comparable (e.g., because the database has been replaced).
