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/Cargo.lock

@@ -9128,6 +9128,7 @@ dependencies = [
  "sc-network-light",
  "sc-network-sync",
  "sc-service",
+ "sc-utils",
  "sp-blockchain",
  "sp-consensus",
  "sp-consensus-babe",