Draft: t2u o2m protocol: Proposed retries

I haven't given this a protocol version yet. I think it's complicated and I don't want to commit to a version yet in case we want to sneak in another protocol update for some reason.

Please LMK if you think this is going to be easy enough to implement, as well of course if you spot edge cases etc. I haven't thought of.

Draft because:

• it needs review
• needs protocol versioning