fork child process for processing job.

This patch is to enable forking of child process for processing the
given job. The forking of child process can be enabled or disabled using
the qonos worker config param flag, 'fork_child_process'. By default
fork_child_process is disabled ie., set to 'False'.
The parent is already registering _terminate so no need to overwrite the registrations. I'll just remove the new registrations and change the signature of stop_processor.