Process pool to replace command execution threads.

Replaces #891 (which, pre cylc-6, targeted the wrong branch)

A process pool to handle execution of job submission, event handler, and poll and kill commands.
Replaces batched command execution in threads. 
damn, it works for me at Python 2.7.5 ... the perils of accessing the module's internals I guess.  Are you at 2.6?  I'll try later at 2.6 on another server...