api: extend Hash RPC to hash by revision

[heyitsanthony]

The Hash RPC hashes all of boltdb. Since hash uses the state of boltdb at the time of the RPC, there must be a quiescent period across the cluster to compute hashes to avoid mismatches from incoming proposals, limiting its usefulness on typical deployments.

To support hashing etcd on a live cluster, the RPC can be extended to include a revision field:

message HashRequest {
    int64 revision = 1;
}

If revision is non-zero, Hash will hash only the keys bucket and only those keys at or below the given revision.

Related: #7125