Batched worker thread for queued task event handlers.

This re-uses the code that handles task job submission. It also tidies
up associated stdout and suite logging a little, and tidies up suite
shutdown (avoid tracebacks on early abort due to attempt shutdown of
uninitialized items, and re-order shutdown of threads and pyro
connections so that the main thread doesn't appear to exit before the
other threads have finished).
Documented event handler queue config.

Also changed the config heading "task event handler execution" to put
"event handler execution", in case we want to manage suite (as opposed
to task) event handlers in the same queue.
Fix plog error - just log will do
Updated tests/events/task ref test.

In the the new event handler queue changes I had removed a log message
that the reference test was relying on.
Merge pull request #256 from hjoliver/event-handler-queue

Batched worker thread for queued task event handlers.
Merge remote branch 'upstream/master' into 140-hostname

Conflicts:
	lib/cylc/task_types/task.py

Use suite_host to get hostname