shutdown upon parentchain code update

[brenzi]

closes #1633

code updates can cause extrinsics to fail unless we can update metadata and extrinsics AdditionalParams.
Our architecture currently doesn't allow to update these dynamically.

Shutting down the service gracefully (assuming automatic restart) is safe and clean and probably doesn't even cause additional downtime as we would need to pause TOP's anyway to avoid race conditions

caveat:

• this shutdown CAN come too late! If a TOP is executed while CodeUpdated is between InBlock and Finalized on the Parentchain, the extrinsic will fail. See shutdown upon parentchain UpdateAuthorized #1635 for a possible workaround and safely unshield using balances.transfer on parentchain #1257 for a proper solution

[clangenb]

Seems like a legit approach to me, given the caveats we are aware of that you mentioned.