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-gossip/src/state_machine.rs

@@ -489,7 +489,7 @@ mod tests {
 			unimplemented!();
 		}
 
-		fn register_notifications_protocol(&self, _: ConsensusEngineId, _: Cow<'static, [u8]>) {}
+		fn register_notifications_protocol(&self, _: ConsensusEngineId, _: Cow<'static, str>) {}
 
 		fn announce(&self, _: B::Hash, _: Vec<u8>) {
 			unimplemented!();