fts: Redesigned/enhanced FTS API and how virtual plugin works with it.

fts: Redesigned/enhanced FTS API and how virtual plugin works with it.
The changes include:

 - Only indexer process and "doveadm index" do FTS indexing now. Other
processes connect to the indexer process via UNIX socket and request
indexing a specific mailbox.

 - FTS backends can now index/search any search key. Current backends
implement indexing for some specific header fields (Subject, From, To, Cc,
Bcc), but it would also be possible to add indexing for e.g. message size
or date.

 - CLucene support is fixed and fully functional.

 - Solr is split to "solr" and "solr_old" backends. The new "solr" backend
supports the new header fields. It also uses mailbox GUIDs as mailbox
identifiers, so that renaming a mailbox doesn't require reindexing.
The "solr_old" uses the old Solr schema and doesn't support any new features.