Support Xapian FTS for Dovecot

As a web hosting provider, I would like Xapian FTS support added to cPanel Dovecot as a more suitable alternative to Solr in the majority of deployments.

Running Solr for Dovecot FTS means suffering the overhead that Java brings.

Dovecot has other FTS drivers, including a soon-to-be-core fts_flatcurve which uses the Xapian library bindings and therefore requires no separate server instance and consequentially no server overhead.

https://doc.dovecot.org/configuration_manual/fts/

https://github.com/slusarz/dovecot-fts-flatcurve/

Providing Xapian as an FTS option would appear to be of minimal change to the cPanel Dovecot distribution since all prerequisites are met and would only need the (simple, with no dependencies, builds cleanly on EL7) Xapian library building and distributing.

Removing the need for Solr, with it's significant overhead, would provide a significant performance gain for systems while maintaining good FTS performance in Dovecot IMAP operations.