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/error.rs

@@ -68,6 +68,15 @@ pub enum Error {
 		/// Name of the protocol registered multiple times.
 		protocol: ProtocolName,
 	},
+	/// Peer does not exist.
+	#[error("Peer `{0}` does not exist.")]
+	PeerDoesntExist(PeerId),
+	/// Channel closed.
+	#[error("Channel closed")]
+	ChannelClosed,
+	/// Connection closed.
+	#[error("Connection closed")]
+	ConnectionClosed,
 }
 
 // Make `Debug` use the `Display` implementation.