fix test task and make it sleep(5)
warn about multiple Servers on the same broker

A Server needs to "own" its broker so that it can keep track of
which jobs exist; Celery provides no *fast* way of finding this out.

TODO it would be even better if we could enforce this somehow.
added /tasklist task for retrieving running and reserved (scheduled) tasks, directly from Celery
added /tasks and /state functions within the server object
added functionality to keep a locally cached list of active tasks within server object
flake8
bug in Server's revoking