Merge branch 'validation-manager-tests' into 'master'

Unit tests for ValidationManagerImpl

Unit test for the validation manager. I also changed the way the validation manager loads unvalidated messages - instead of using a single DB task to load all unvalidated messages, it loads a list of message IDs and then loads each message in a separate task. This prevents the DatabaseExecutor from being blocked by a long-running task if there are lots of messages to validate.

See merge request !113