Support Efficient Iteration of ConsensusStates in Reverse Chronological Order

[AdityaSripal]

SDK should provide efficient iteration of consensus states for a particular client in reverse chronological order.

To accomplish this, the consenus state height must be stored as part of the key in such a way that the reverse lexicographic order is the same as the (reverse) chronological order. Then we could simply use the SDK store's ReverseIterator implementation.

[colin-axner]

I believe this is ibc-breaking now that ibc 1.0 has been released since this would change proof paths

[colin-axner]

#125 implements this at the 07-tendermint level, but every other light client would need to reimplement this logic to get support for pruning and other mechanisms which require efficient iteration.

This is IBC breaking and should only be applied at the 02-client level once we are ready to move on to IBC v2