[FTS Xapian] Beta release

Hi

Please find attached the beta release of FTS Xapian, with the objective to replace fts_squat that is being deprecated.

Configuration is exactly the same as for fts_squat:

plugin {

plugin = fts fts_xapian (...) fts = xapian fts_autoindex = yes fts_enforced = yes fts_xapian = partial=2 full=20

This is installed on my production server (>120Gb of mailboxes), and I will observe it during the coming days.

I will definitely appreciate that this is added in the core git of docevot, in order to have a versionning of it, to remove squat and let basic users able to avoid Solr alternative as much as possible.

Thanks

JM