This adds some initial code for the nostrdb relay subscription monitor.
When new notes are written to the database, they are checked against
active subscriptions. If any of the subscriptions are matched, the note
primary key is written to the inbox queue for that subscription.
We also add an ndb_wait_for_notes() method that simply waits for notes
to be written by the subscription monitor.
Changelog-Added: Added filter subscriptions
Signed-off-by: William Casarin <jb55@jb55.com>
