After some reflection and discussion with Ben, re-introduced the
RangeID into the proto.Replica description. We decided it would be
a mistake to lose an exact, unique description of a particular range
replica, which we get via proto.Replica. It consists of three IDs
which together are a unique identifier: NodeID, StoreID, RangeID.

To help with debugging and make things simpler to reason about,
introduced a new "RaftID" which is defined at the level of
proto.RangeDescriptor.