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'.
Say that there is an exception during _run_loop, is it true that if the exception was related to a SIGTERM or SIGINT then this signal handler would run before _child_pid is set to None?