Allow Substrate Pallet to be Halted (#485)

* Copy-Pasta owner and freezing code from `message-lane`

* Halt pallet if bridge hasn't been initialized

* Make owner optional in `message-lane` pallet

* Add `is_halted` to `InitializationData`

* Fix initialization tests

* Only allow pallet to be initialized once

* Add some logging around halting and ownership changes

* Remove `target` in debugging calls

bridges/bin/millau/node/src/chain_spec.rs

@@ -158,6 +158,7 @@ fn testnet_genesis(
 		pallet_substrate_bridge: Some(BridgeRialtoConfig {
 			// We'll initialize the pallet with a dispatchable instead.
 			init_data: None,
+			owner: None,
 		}),
 		pallet_sudo: Some(SudoConfig { key: root_key }),
 		pallet_session: Some(SessionConfig {