Performance improvement on FLAGS-only FETCH

* Compute the intersection set of the uids on a FETCH, so we avoid
  iterating through the non-existant UIDs.
* Dispatch FLAGS query to our specialized method, that fetches all the
  flags documents and return objects that only specify one subset of the
  MessagePart interface, apt to render flags quickly with less queries
  overhead.
* Overwrite the do_FETCH command in the imap Server to use fetch_flags.
* Use deferLater for a better dispatch of tasks in the reactor.
Update service initialization file

This will need to place a configuration file with:

* userid
* uuid
* password (optional)

Use it for even faster startup times, and running under the native
twisted reactor.