InitChain allows the app to return the initial validator set. This is nice because it allows the initial validator set to be a function of application state, for instance some set of bonds/delegations.

However, Tendermint currently requires the genesis to have at least one validator. We should loosen this requirement, but then ensure that the validator set after InitChain has at least one validator ...

Note we create the state before calling InitChain and then update it after InitChain.