Merge branch 'message-tracker-refactoring' into 'master'

Factor MessageTracker out of BdfIncomingMessageHook

The branch moves the MessageTracker implementation from BdfIncomingMessageHook to a separate class. This will allow the private group invitation client to track messages from classes other than the one that implements the delivery hook.

I've also fixed a couple of bugs, removed some redundant code from the validation manager, and added null-safety annotations (which is how I noticed the bugs).

See merge request !405