discovery+routing: reduce number of active validation barrier jobs

In order to reduce high CPU utilization during the initial network view
sync, we slash down the total number of active in-flight jobs that can
be launched.

routing/router.go

@@ -518,7 +518,7 @@ func (r *ChannelRouter) networkHandler() {
 
 	// We'll use this validation barrier to ensure that we process all jobs
 	// in the proper order during parallel validation.
-	validationBarrier := NewValidationBarrier(runtime.NumCPU()*10, r.quit)
+	validationBarrier := NewValidationBarrier(runtime.NumCPU()*4, r.quit)
 
 	for {
 		select {