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/network/src/behaviour.rs

@@ -255,7 +255,7 @@ impl<B: BlockT, H: ExHashT> Behaviour<B, H> {
 	pub fn register_notifications_protocol(
 		&mut self,
 		engine_id: ConsensusEngineId,
-		protocol_name: impl Into<Cow<'static, [u8]>>,
+		protocol_name: impl Into<Cow<'static, str>>,
 	) {
 		// This is the message that we will send to the remote as part of the initial handshake.
 		// At the moment, we force this to be an encoded `Roles`.