client/*: Treat protocol name as str and not [u8] (#6967)

* client/*: Treat protocol name as str and not [u8]

Notification protocol names are in practice always valid utf8 strings.
Instead of treating them as such in the type system, thus far they were
casted to a [u8] at creation time.

With this commit protocol names are instead treated as valid utf8
strings throughout the codebase and passed as `Cow<'static, str>`
instead of `Cow<'static, [u8]>`. Among other things this eliminates the
need for string casting when logging.

* client/network: Don't allocate when protocol name is borrowed

substrate/client/finality-grandpa/src/communication/tests.rs

@@ -61,7 +61,7 @@ impl sc_network_gossip::Network<Block> for TestNetwork {
 		let _ = self.sender.unbounded_send(Event::WriteNotification(who, message));
 	}
 
-	fn register_notifications_protocol(&self, _: ConsensusEngineId, _: Cow<'static, [u8]>) {}
+	fn register_notifications_protocol(&self, _: ConsensusEngineId, _: Cow<'static, str>) {}
 
 	fn announce(&self, block: Hash, _associated_data: Vec<u8>) {
 		let _ = self.sender.unbounded_send(Event::Announce(block));