SW-816: implemented truncate trigger which cascades a truncate on resource_claim to resource_usage and resource_usage_delta

SW-816: improved speed of the RADBCommonTestMixin by only creating a postgres instance upon setUpClass, and applying a fresh radb schema every test via setUp

Resolve SW-815

See merge request !55

SW-816: bonus feature: refactored RADBCommonTest into a RADBCommonTestMixin which takes care of setting up a testing postgres radb instance, and which can be used in conjunction with a unittest.TestCase to do unittests on a fresh RADB instance. By using a mixin instead of chained inheritance we have better seperation of responsibility, and tools like pycharm can handle these tests better as well.

SW-816: modified radb insertSpecificationAndTask into insertOrUpdateSpecificationAndTask. This has the advantage that we do not have to reconnect predecessor relations, and in the webscheduler a re-insert of a task will now be visible as an update on an existing task, instead of a delete (disappear) followed by an insert (reappear). Catch task_predecessor_unique PostgresDBQueryExecutionError in the Specification class of the resourceassigner, so scheduling can continue even on tasks which were already known and already had links to predecessors.

SW-816: added test test_do_assignment_does_not_raise_on_inserting_predecessors which reproduces the bug of SW-816. It fails with a 'duplicate key value violates unique constraint task_predecessor_unique' error from the RABD when scheduling a successor task for the second time.

SW-816: rewrote t_resourceassigner to use a testing-RADB, thereby removing many wrong assumptions made in the mocks. The resource_assgner and its schedulers can still be simplified significantly if we just use the RADB as designed; just insert the claims, and let the RADB check/set the conlict status.

ROHD-2065: fixed typos

See merge request !56

Resolve SW-811

See merge request !51

SW-811: ToBus.send now retries to reconnect and resend the message upon connection loss. Added test for that as well.

SW-811: added test to prove that sending a message on a broken connection fails with a MessagingError

SW-810: renamed unused-IS614* to LV614*

See merge request !49

SW-796: removed obsolete kombu-based test for qpid-based MessageRouter. The MessageRouter is a deprecated qpid tool that should be removed in the future. If also mimics behaviour that a proper qpid and/or rabbitmq broker can do, so there is even not need for this tool. It is too much work to remove it at this moment though, so we'll leave the system as it is.