We should be able to inspect the current GRANDPA round state externally, this is particularly useful in situations where finality is stalled (e.g. due to some voters being offline).

I propose we add an RPC method grandpa_roundState that returns something like:

{
  "setId": 361,
  "round": 1283,
  "total_weight": 100,
  "thresholdWeight": 67,
  "prevotes": {
    "currentWeight": 79,
    "missing": [
      // list of addresses of authorities missing votes
    ]
  },
  "precommits": {
    "currentWeight": 12,
    "missing": [
      // list of addresses of authorities missing votes
    ]
  }
}

Currently this data is not exposed by the finality-grandpa crate, there's paritytech/finality-grandpa#55 which should be tackled as part of this issue as well.

Regarding the addresses of validators that are missing votes it is a bit tricky. Ideally we'd like to expose the addresses of the stash account since these represent the main "identity" of a validator. But the client GRANDPA code only deals with session keys and it isn't trivial to fetch the associated stash keys.

cc @rphmeier