Full text search with fts_xapian

Goal: being able to search for email with some text in the body in finite time

This uses fts_xapian to create an index of mail content. This can be resource intensive, especially during initial indexation.

An option is offered to store indices elsewhere, because there is no point in backing up indices (which can grow large).

A reasonable amount of tests has been added. When indexing is enabled, I get a warning during the test which never fires in production (I have been using a variation of this setup for months now). I asked on the mailing list and noone answered: https://dovecot.org/pipermail/dovecot/2020-August/119575.html

The git history is full of junk, but I'll only clean it up after some positive review.