Importing zerver.worker.queue_processors (which is needed to get the list of
workers to start) is slow because it, in turn, imports a bunch of stuff. So we
move the process of starting up queue processing workers into another script
that gets started in parallel with everything else.
(imported from commit 839bada6dc7b93825c69b0d8fd9fbe2de75eabee)
Zev Benjamin