Give all replicas of a range the same Range ID

Previously, the range ID was chosen as unique / store for every new range
replica created on a store. This means that there was previously no way
to refer to a Raft consensus group by ID. The only way would have been to
use StartKey->EndKey.

Now, ranges are allocated from a global id allocator in blocks by stores.
When new ranges are created, an ID is taken from the pre-alloc'd block.

Range ID has now been moved out of proto.Replica and moved into RangeDescriptor.
Replica no only specifies NodeID and StoreID, which is enough information to
locate the device where a replica of the range can be found.

RangeID was not needed in RangeMetadata, so was removed from there.
Also, ZoneConfig.Replicas was renamed ZoneConfig.ReplicaAttrs to eliminate
confusion.
add a TODO? since this need to be configurable?