indexer: Added OPTIMIZE command to optimize fts index.

doveadm index: Added -n parameter to specify max number of recent msgs to perform indexing. The idea is that if a user hasn't opened the mailbox for a long time (or ever), there's not much point in wasting time on doing indexing.

Added "indexer" service, which uses worker processes to perform queued mailbox indexing. Only a single worker process will index the same user at the same time. This avoids lock waits, especially when doing full text search indexing with backends that require locking.