[Dovecot] Full text search improvements

[Mike Abbott]

> how [FTS indexing] could be improved for everyone in future

For sites which set client_limit > 1 it would help performance not to stall for INDEXER_WAIT_MSECS when polling the indexer for input.  Currently dovecot unwinds back out to the main command loop repeatedly to allow other clients to use the process but it also stalls the whole process for INDEXER_WAIT_MSECS every time it finds no input from the indexer, which hurts responsiveness for those other clients.  This can be avoided by removing the client's I/O from the main ioloop and adding the indexer's instead, or perhaps by leveraging CLIENT_COMMAND_STATE_WAIT_EXTERNAL.

Third-party FTS implementations may benefit from having the NOT/AND/OR seq_range_array merging logic in squat_lookup_arg() generalized and made available to all.

It would also be helpful if FTS expunge were asynchronous, but this is not critical.