Better support for the IsSystemMessage metadata tag

Relates to #429
SentTime no longer required

Also added tests for missing IsSystemMessage
For Integration Scenario

Messages without IsControlMessage or EnclosedMessageTypes headers will still be processed and wont cause exceptions in SC
Relates to https://github.com/Particular/ServiceControl/issues/429
Fixed incorrect assert on acceptance test.
Merge branch 'hotfix-1.4.3'