This commit does two things:
- removes the concept of allow / deny. Having this in place was a
minor optimization and removing it makes the solution simpler.
- changes the job dependency tracking to track sets of abstact
parent jobs rather than individual parent jobs.
As a note, the purpose of the ValidationBarrier is that it allows us
to launch gossip validation jobs in goroutines while still ensuring
that the validation order of these goroutines is adhered to when it
comes to validating ChannelAnnouncement _before_ ChannelUpdate and
_before_ NodeAnnouncement.