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.

lib-storage: Plugin API changed to run plugin functions in correct order. Previously the hooks were run in correct order, but the functions they overrode were run in reverse order. This caused problems when multiple plugins were used. --HG-- branch : HEAD

lib-storage: Plugin hook API changed to support per-user plugins within a process. --HG-- branch : HEAD

Mailbox opening and closing APIs changed. --HG-- branch : HEAD

Changed .h ifdef/defines to use <NAME>_H format. --HG-- branch : HEAD

Added full text search plugin framework. Still missing support for handling expunges. Also it could use better logic with more complex SEARCH queries to figure out when the FTS index should be used. --HG-- branch : HEAD