match messages from the full-text index by rowid

"uuid" is a primary key in "messages" but not in "messages_index",
the implication of that is very slow matching by UUID.

What can be done instead is matching messages_index.rowid to messages.rowid,
that is, an always-present clustered index.

This not only improves performance of full-text search but also of just
updating messages in any shape or form.