Interop: Message Expiry

It would be nice to be able to recover messages when they are sent to remote chains that are not able to process them. There are generally 2 ways of thinking about this. Messages can be expired after some amount of time and be rolled back or they can revert up front if they are going to a chain that cannot accept the incoming message.

Start by defining how messages sent to remote chains that cannot process them should be handled, comparing expiry with upfront rejection. Done requires an agreed specification for recovery or rollback behavior; the issue names no files, tests, or entry points to inspect.

Written by the indexing model from the issue text.