Minor code loop improvements

These small changes are mainly cosmetic, although can shave 1 sec off execution time for a trivial suite :-)

I was looking at use of ```time.sleep()``` to see why a suite with one task for ```/bin/true``` now takes 6 sec instead of the ~1 sec that I vaguely recall pre cylc-5 (the cause is a sleep in the batch submission thread(s) main loop, which is necessary to keep CPU usage down).

@matthewrmshin et.al. - please review