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'.
See the comment I left above - I had missed seeing that this method was being registered directly as a handler, but I don't know that it should be anyway rather than funneling through the existing Worker._terminate() method as in the single process approach.