database: Rename internal find_messages_by_term to find_doc_ids

database: Rename internal find_messages_by_term to find_doc_ids

This name is a more accurate description of what it does, and
the more general naming will make sense as we start storing
non-message documents in the database (such as directory
timestamps).

Also, don't pass around a Xapian::Database where it's more our
style to pass a notmuch_database_t*.