Redesigned mail precaching APIs. There's no longer a one monolithic mailbox_sync(MAILBOX_SYNC_FLAG_PRECACHE) call, but rather one mail_precache() call for each mail to be precached. This allows the callers to show the progress and in general is cleaner.

fts: Log how many messages were indexed.

fts: Strip text/html mails to plaintext before sending them to FTS backend.

fts: Convert input text to decomposed titlecase only for Squat backend. Lucene and Solr can handle case-insensitivity internally.

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.