network/consensus: use new dbft StopTxFlow callback

It makes sense in general (further narrowing down the time window when
transactions are processed by consensus thread) and it improves block times a
little too, especially in the 7+2 scenario.

Related to #2744.

pkg/network/server_test.go

@@ -461,6 +461,7 @@ func TestTransaction(t *testing.T) {
 	cons := new(fakeConsensus)
 	s.AddConsensusService(cons, cons.OnPayload, cons.OnTransaction)
 	startWithCleanup(t, s)
+	s.RequestTx(util.Uint256{1})
 
 	t.Run("good", func(t *testing.T) {
 		tx := newDummyTx()