For async. concordance calculation, report timeouted Rq task and remove unfinished concordance

[tomachalek]

I.e. in case _should_be_bg_query (see search.py) says True and the calculation goes to an Rq worker and the client just regularly checks for status (either via Ajax or Websockets), the checking routine should be able to detect timeouted task and make sure the unfinished concordance is removed and the client is informed that the calculation failed.

To test and develop that, it will probably be necessary to temporarily change things like Rq task timeout, _should_be_bg_query (to always return True).