parachain-system (#296)

* rename parachain-{upgrade -> system}

* Merge message-broker into parachain-system

* Remove message-broker and clean up

* Update docs

* Test upward messages sending

And also update the relay-sproof-builder so that it allows to set the
relay dispatch queue size for the given parachain.

* Test horizontal message sending

* Remove old inherent definitions

cumulus/rococo-parachains/pallets/parachain-info/src/lib.rs

@@ -32,7 +32,7 @@ impl<T: Config> Get<ParaId> for Module<T> {
 }
 
 decl_storage! {
-	trait Store for Module<T: Config> as ParachainUpgrade {
+	trait Store for Module<T: Config> as ParachainInfo {
 		ParachainId get(fn parachain_id) config(): ParaId = 100.into();
 	}
 }