fix jedi completions request

Current threads implementation in Jedi completer makes an event loop in ``SetCandidates`` method which waits for a ``query_ready`` flag to set.

This means that this thread is always alive, so ``self._completion_thread.is_alive()`` always returns ``True`` and else clause never runs ( which is good, because if it will run it will make more and more daemon threads because of ``_start_completion_thread() `` call).

So, obvious fix is to just return ``Set`` flag of ``_candidates_ready`` event which is always set when thread done all the work and reset on CandidatesForQuery method call. Also it should speed things up a little