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, I don't know how I did that ... another instance of https://github.com/cylc/cylc/pull/1022#discussion_r15044098