Attempt to relieve pressure on mpsc_network_worker (#13725)

* Attempt to relieve pressure on `mpsc_network_worker`

`SyncingEngine` interacting with `NetworkWorker` can put a lot of strain
on the channel if the number of inbound connections is high. This is
because `SyncingEngine` is notified of each inbound substream which it
then can either accept or reject and this causes a lot of message
exchange on the already busy channel.

Use a direct channel pair between `Protocol` and `SyncingEngine`
to exchange notification events. It is a temporary change to alleviate
the problems caused by syncing being an independent protocol and the
fix will be removed once `NotificationService` is implemented.

* Apply review comments

* fixes

* trigger ci

* Fix tests

Verify that both peers have a connection now that the validation goes
through `SyncingEngine`. Depending on how the tasks are scheduled,
one of them might not have the peer registered in `SyncingEngine` at which
point the test won't make any progress because block announcement received
from an unknown peer is discarded.

Move polling of `ChainSync` at the end of the function so that if a block
announcement causes a block request to be sent, that can be sent in the
same call to `SyncingEngine::poll()`.

---------

Co-authored-by: parity-processbot <>

substrate/client/network/test/src/sync.rs

@@ -414,7 +414,7 @@ async fn can_sync_small_non_best_forks() {
 	// poll until the two nodes connect, otherwise announcing the block will not work
 	futures::future::poll_fn::<(), _>(|cx| {
 		net.poll(cx);
-		if net.peer(0).num_peers() == 0 {
+		if net.peer(0).num_peers() == 0 || net.peer(1).num_peers() == 0 {
 			Poll::Pending
 		} else {
 			Poll::Ready(())