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