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/common/src/role.rs

@@ -28,7 +28,7 @@ use codec::{self, Encode, EncodeLike, Input, Output};
 /// > **Note**: This enum is different from the `Role` enum. The `Role` enum indicates what a
 /// >			node says about itself, while `ObservedRole` is a `Role` merged with the
 /// >			information known locally about that node.
-#[derive(Debug, Clone)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub enum ObservedRole {
 	/// Full node.
 	Full,
@@ -45,6 +45,18 @@ impl ObservedRole {
 	}
 }
 
+impl From<Roles> for ObservedRole {
+	fn from(roles: Roles) -> Self {
+		if roles.is_authority() {
+			ObservedRole::Authority
+		} else if roles.is_full() {
+			ObservedRole::Full
+		} else {
+			ObservedRole::Light
+		}
+	}
+}
+
 /// Role of the local node.
 #[derive(Debug, Clone)]
 pub enum Role {