Update git submodule.
The worker should remove itself from the workers list on shutdown
worker uses new _deregister_worker lua command to remove itself on termination.
fix typo
update to deregister_workers lua script is.

Refactor test according to suggestions from @myronmarston
Fix worker de-registration so it works against latest master.

- The script name changed (deregister_worker => deregister_workers)
- Add a better interface (Qless::Client#deregister_workers).
- Deal with the fact that one worker can be talking to many clients.
Merge branch 'databus23-remove_worker_on_shutdown'