Improvements to the CeleryNodeMonitor

Improved the responsiveness of the monitoring thread and moved the CeleryNodeMonitor tests to their own module. Now the thread checks every 0.3 seconds if the main job ended and exits early in the regular case, while at it checks for node failure only every 30 seconds.

Tests are running: https://ci.openquake.org/job/zdevel_oq-engine/356