Review design of IBC rewards distribution

[ethanfrey]

We currently use a ConsumerPacket::DistributeRewards packet to send the tokens from consumer to distribute to the stakers on the provider. We need to review the semantics of that.

First, we need to allow multiple such packets in parallel, as we will want to distribute to all validators at the epoch. We may make one batch packet eg DistributeRewards (see #77) to match the two API calls on converter, which will do all work in one packet.

However, we also need some logic for rollback / commit on error / success, which is missing. It is unclear what needs to be done in an error case. How do we account for those tokens?