lib: optimize counting documents

From #xapian

olly> bremner: btw, i noticed notmuch count see ms to request all the documents and then ignores them

bremner> hmm. There's something funny about the way that notmuch uses matches in general iirc

olly> it should be able to do: mset = enquire.get_mset (0, 0, notmuch->xapian_db->get_doccount ());
...
olly> get_matches_estimated() will be exact because check_at_least is the size of the database