Multi-processing process pools for job submission, event handlers, poll-and-kill 

This replaces batched parallel job execution in dedicated single threads with proper multiprocessing pools; addresses #596.

My test battery passes.

Not for merge just yet - a bit still to do: 
   * some tidying
   * handle all exceptions properly, e.g. for every possible way a job submission can fail
   * configurable pool sizes etc.; and obsoletion of the old batch config