Does the validator set really need to be copied here? https://github.com/tendermint/tendermint/blob/master/consensus/state.go#L752

Would be nice to have a better way to get the index here: https://github.com/tendermint/tendermint/blob/master/consensus/common_test.go#L137

Race caught here: https://circleci.com/gh/tendermint/tendermint/189