mail/dovecot-fts-xapian: Update to 1.9.3

This update adds support for dovecot 2.4, allows to limit the amount
of threads via the maxthreads option and fixes several bugs.
For 2.3, e. g. optionally use:
plugin {

fts = xapian
fts_xapian = lowmemory=500 maxthreads=4 partial=3 verbose=0
fts_autoindex = yes
fts_enforced = yes
<...>

}
Approved by: otis (maintainer)
Differential Revision: https://reviews.freebsd.org/D55911