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/network/src/lib.rs

@@ -244,7 +244,6 @@
 
 mod behaviour;
 mod protocol;
-mod service;
 
 #[cfg(test)]
 mod mock;
@@ -258,25 +257,30 @@ pub mod peer_info;
 pub mod peer_store;
 pub mod protocol_controller;
 pub mod request_responses;
+pub mod service;
 pub mod transport;
 pub mod types;
 pub mod utils;
 
-pub use event::{DhtEvent, Event, SyncEvent};
+pub use event::{DhtEvent, Event};
 #[doc(inline)]
 pub use libp2p::{multiaddr, Multiaddr, PeerId};
 pub use request_responses::{Config, IfDisconnected, RequestFailure};
-pub use sc_network_common::{role::ObservedRole, types::ReputationChange};
+pub use sc_network_common::{
+	role::{ObservedRole, Roles},
+	types::ReputationChange,
+};
 pub use service::{
 	signature::Signature,
 	traits::{
-		KademliaKey, NetworkBlock, NetworkDHTProvider, NetworkEventStream, NetworkNotification,
-		NetworkPeers, NetworkRequest, NetworkSigner, NetworkStateInfo, NetworkStatus,
-		NetworkStatusProvider, NetworkSyncForkRequest, NotificationSender as NotificationSenderT,
-		NotificationSenderError, NotificationSenderReady,
+		KademliaKey, MessageSink, NetworkBlock, NetworkDHTProvider, NetworkEventStream,
+		NetworkNotification, NetworkPeers, NetworkRequest, NetworkSigner, NetworkStateInfo,
+		NetworkStatus, NetworkStatusProvider, NetworkSyncForkRequest,
+		NotificationSender as NotificationSenderT, NotificationSenderError,
+		NotificationSenderReady, NotificationService,
 	},
-	DecodingError, Keypair, NetworkService, NetworkWorker, NotificationSender, NotificationsSink,
-	OutboundFailure, PublicKey,
+	DecodingError, Keypair, NetworkService, NetworkWorker, NotificationSender, OutboundFailure,
+	PublicKey,
 };
 pub use types::ProtocolName;