discovery+graph: track job set dependencies in ValidationBarrier

Prior to this commit, it was rare, but possible that proper
validation order was not adhered to when using the ValidationBarrier.
This commit does two things that fix this:
- 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 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.

discovery/gossiper.go

@@ -1470,11 +1470,18 @@ func (d *AuthenticatedGossiper) networkHandler() {
 			// We'll set up any dependent, and wait until a free
 			// slot for this job opens up, this allow us to not
 			// have thousands of goroutines active.
-			validationBarrier.InitJobDependencies(announcement.msg)
+			annJobID, err := validationBarrier.InitJobDependencies(
+				announcement.msg,
+			)
+			if err != nil {
+				announcement.err <- err
+				continue
+			}
 
 			d.wg.Add(1)
 			go d.handleNetworkMessages(
-				announcement, &announcements, validationBarrier,
+				announcement, &announcements,
+				validationBarrier, annJobID,
 			)
 
 		// The trickle timer has ticked, which indicates we should
@@ -1525,7 +1532,8 @@ func (d *AuthenticatedGossiper) networkHandler() {
 //
 // NOTE: must be run as a goroutine.
 func (d *AuthenticatedGossiper) handleNetworkMessages(nMsg *networkMsg,
-	deDuped *deDupedAnnouncements, vb *graph.ValidationBarrier) {
+	deDuped *deDupedAnnouncements, vb *graph.ValidationBarrier,
+	jobID graph.JobID) {
 
 	defer d.wg.Done()
 	defer vb.CompleteJob()
@@ -1536,7 +1544,7 @@ func (d *AuthenticatedGossiper) handleNetworkMessages(nMsg *networkMsg,
 
 	// If this message has an existing dependency, then we'll wait until
 	// that has been fully validated before we proceed.
-	err := vb.WaitForDependants(nMsg.msg)
+	err := vb.WaitForParents(jobID, nMsg.msg)
 	if err != nil {
 		log.Debugf("Validating network message %s got err: %v",
 			nMsg.msg.MsgType(), err)
@@ -1566,7 +1574,7 @@ func (d *AuthenticatedGossiper) handleNetworkMessages(nMsg *networkMsg,
 
 	// If this message had any dependencies, then we can now signal them to
 	// continue.
-	vb.SignalDependants(nMsg.msg, allow)
+	vb.SignalDependents(nMsg.msg, jobID)
 
 	// If the announcement was accepted, then add the emitted announcements
 	// to our announce batch to be broadcast once the trickle timer ticks