action queue: Added debug log messages.

Signed-off-by: Joe MacMahon <joe.macmahon@cern.ch>
tests: Updated common methods to do debug output more effectively

Signed-off-by: Joe MacMahon <joe.macmahon@cern.ch>
tests: Wrote some tests for async/action queue

* Test for flushing operation
* Test for auto-flushing on an interval
* Test for auto-flushing on hitting the max_queue_length

Signed-off-by: Joe MacMahon <joe.macmahon@cern.ch>