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'.
Why not register the Worker._terminate() method has the handler here? Should work just the same in the context of the child process as it does in the context of a single (non-forked) process, right? Then the stop_processor wouldn't be forced to implement the signal handler interface. Also, the _terminate method has been modified (Lines 230 - 234) in this PR as to work in that scenario, so I'm wondering if that was the intention in the first place or is an unfinished change.