Rework the event system of sc-network (#1370)

This commit introduces a new concept called `NotificationService` which
allows Polkadot protocols to communicate with the underlying
notification protocol implementation directly, without routing events
through `NetworkWorker`. This implies that each protocol has its own
service which it uses to communicate with remote peers and that each
`NotificationService` is unique with respect to the underlying
notification protocol, meaning `NotificationService` for the transaction
protocol can only be used to send and receive transaction-related
notifications.

The `NotificationService` concept introduces two additional benefits:
  * allow protocols to start using custom handshakes
  * allow protocols to accept/reject inbound peers

Previously the validation of inbound connections was solely the
responsibility of `ProtocolController`. This caused issues with light
peers and `SyncingEngine` as `ProtocolController` would accept more
peers than `SyncingEngine` could accept which caused peers to have
differing views of their own states. `SyncingEngine` would reject excess
peers but these rejections were not properly communicated to those peers
causing them to assume that they were accepted.

With `NotificationService`, the local handshake is not sent to remote
peer if peer is rejected which allows it to detect that it was rejected.

This commit also deprecates the use of `NetworkEventStream` for all
notification-related events and going forward only DHT events are
provided through `NetworkEventStream`. If protocols wish to follow each
other's events, they must introduce additional abtractions, as is done
for GRANDPA and transactions protocols by following the syncing protocol
through `SyncEventStream`.

Fixes https://github.com/paritytech/polkadot-sdk/issues/512
Fixes https://github.com/paritytech/polkadot-sdk/issues/514
Fixes https://github.com/paritytech/polkadot-sdk/issues/515
Fixes https://github.com/paritytech/polkadot-sdk/issues/554
Fixes https://github.com/paritytech/polkadot-sdk/issues/556

---
These changes are transferred from
https://github.com/paritytech/substrate/pull/14197 but there are no
functional changes compared to that PR

---------

Co-authored-by: Dmitry Markin <dmitry@markin.tech>
Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>

substrate/client/mixnet/src/protocol.rs

@@ -18,7 +18,10 @@
 
 use super::config::Config;
 use mixnet::core::PACKET_SIZE;
-use sc_network::{config::NonDefaultSetConfig, ProtocolName};
+use sc_network::{
+	config::{NonDefaultSetConfig, NonReservedPeerMode, SetConfig},
+	NotificationService, ProtocolName,
+};
 
 /// Returns the protocol name to use for the mixnet controlled by the given chain.
 pub fn protocol_name(genesis_hash: &[u8], fork_id: Option<&str>) -> ProtocolName {
@@ -31,12 +34,26 @@ pub fn protocol_name(genesis_hash: &[u8], fork_id: Option<&str>) -> ProtocolName
 }
 
 /// Returns the peers set configuration for the mixnet protocol.
-pub fn peers_set_config(name: ProtocolName, config: &Config) -> NonDefaultSetConfig {
-	let mut set_config = NonDefaultSetConfig::new(name, PACKET_SIZE as u64);
+pub fn peers_set_config(
+	name: ProtocolName,
+	config: &Config,
+) -> (NonDefaultSetConfig, Box<dyn NotificationService>) {
+	let (mut set_config, service) = NonDefaultSetConfig::new(
+		name,
+		Vec::new(),
+		PACKET_SIZE as u64,
+		None,
+		SetConfig {
+			in_peers: 0,
+			out_peers: 0,
+			reserved_nodes: Vec::new(),
+			non_reserved_mode: NonReservedPeerMode::Deny,
+		},
+	);
 	if config.substrate.num_gateway_slots != 0 {
 		// out_peers is always 0; we are only interested in connecting to mixnodes, which we do by
 		// setting them as reserved nodes
 		set_config.allow_non_reserved(config.substrate.num_gateway_slots, 0);
 	}
-	set_config
+	(set_config, service)
 }