celeryd hanging on restart/stop

Added configuration option CELERYD_KILL_DELAY. When this is set celeryd will wait the specified number of seconds and any workers that have not restarted/stopped will be killed to then be processed as needed.

Certain processes running were not being terminated by a SIGTERM signal and thus added this. 

Perhaps there is/could be a better way of terminating workers not responding to a 'shutdown' broadcast?