fts-solr: Add and use FTS_SOLR_USER_CONTEXT_REQUIRE Satisfies static analyzers

fts-solr: fts_solr=use_libfts send data to Solr via space-separated tokens. In this case Solr should be configured to not do any kind of filtering and use only WhitespaceTokenizerFactory.

fts-solr: Fixed support for multiple Solr hosts when same process handles multiple users. E.g. indexer-worker was always using only the first user's Solr host.

fts-solr: Deinit solr connection at plugin unload.

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.

fts: Replaced unnecessary lock()/unlock() from API with refresh()

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

plugins: Added dependencies. The dependencies can only be checked if the dlopen() can successfully open the module, so use (apparently) non-portable ltlibrary linking to force the libraries we depend on to load with us. --HG-- branch : HEAD

Fixed plugins to work with config rewrite. --HG-- branch : HEAD

fts: Added support for handling multiple namespaces. --HG-- branch : HEAD

fts-solr: Added break-imap-search setting to use Solr with SEARCH TEXT/BODY parameters. This makes the SEARCH non-compliant with standard IMAP protocol, because TEXT/BODY require support for substring searches and Lucene doesn't support them. --HG-- branch : HEAD

Initial code to support Apache Solr (Lucene indexing server). --HG-- branch : HEAD