Uninitialized Replicas are currently just a Replica with an incomplete RangeDescriptor. However, nearly all methods on uninitialized replicas are invalid and result in often-subtle failures like #6027. We should create a separate uninitializedReplica type which only contains a raftGroup and converts to a full replica when it gets its snapshot. The raft-related functionality in Store will need to change to look in both Store.replicas and Store.uninitReplicas instead of assuming the latter to be a subset of the former.