jobs.py: Reduce FD leaks from queues and process objects

The garbage collector can take too long to get around to cleaning
up the Queue and Process instances in completed Job instances.  As
such, FDs tend to leak and in very large projects this can result
in running out of FDs before a build, fetch, track, or other process
can complete.  This patch reduces the chance of that by only creating
the queue when it's needed, and forcing the queue and process instances
to be deleted when the parent is finished with them.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>