Allow for busy waiting

[patricksnape]

Allow a worker to sleep and thus busy wait if it finds no jobs, rather than exiting upon completion.

[jabooth]

This could be controlled by a Boolean stored in the database:

if db.should_busywait():
    sleep(db.busywait_pause_time())
else:
    # we are done!

That way you can bring down a fleet of busywait workerbees, or adjust wait duration, on the fly.