[Pulp-dev] Port Pulp3 to use RQ

[Jeremy Audet]

> RQ does not support revoking tasks.  If you send the worker a SIGINT, it will finish the task and then stop processing new ones.  If you send the worker SIGKILL, it will stop immediately, but I don't think it gracefully handles this circumstance.

Nothing handles SIGKILL gracefully. Processes can't catch that signal.
`kill -9 $pid` sends SIGKILL.

If one is looking for a way to gracefully, immediately kill an RQ
worker, then SIGTERM may do the trick. Anecdotally, many processes
handle this signal in a hurried fashion. Semantically, this is
appropriate: SIGINT is the "terminal interrupt" signal (Ctrl+c sends
SIGINT), whereas SIGTERM is the "termination signal."