Track queue worker pids and kill on exit

When spawning the queue worker processes, keep a list of their process IDs
and write them to the pidfile.

When exiting, send SIGKILL to the queue worker processes.
Merge pull request #98 from rschlaikjer/kill-queue-workers-on-exit

Track queue worker pids and kill on exit

All workers and threads need to be managed by pushmanager so they can be killed properly.