Support loopback

[cwgoes]

Ref https://github.com/cosmos/ics/issues/138#issuecomment-509112606

Channels created with zero connection hops should loop-back to the same chain. No proofs then need to be checked.

[cwgoes]

For now, should this be kept asynchronous? Otherwise we have to alter the interface.

Would result in temporary store usage, but proofs could be omitted.

[cwgoes]

...doing this correctly is nontrivial, it requires adding a bunch of switches

[mossid]

Which ICSs have to be worked to implement loopback? I guess ICS 23 and 02, and we can reuse all the above?

[cwgoes]

Which ICSs have to be worked to implement loopback? I guess ICS 23 and 02, and we can reuse all the above?

It has to be concretely efficient; I guess we could substitute out the usual Merkle proof verification for a dummy one which just checks the store.

[cwgoes]

I think this has to be considered jointly with the solo-solo / solo-chain / chain-chain question.

One option is to rework the abstractions around commitments and have clients expose a state verification directly instead of assuming commitment proofs and passing those around. Then solo machine clients could use signatures, blockchains could use Merkle proofs, and loopback could just read from state directly.

[cwgoes]

^ This had better be done soon, I still agree with the plan.