Remove range ID and use raft ID instead.

The idea is we'll combine Node ID and Store ID (both int32) into a
single Raft Node ID (int64). We no longer need the extra 64 bit range
ID as it's obviated by simply utilizing the Raft ID.

There are some problems with this scheme. In particular, The same Raft
group might appear twice on the same store over time. We want to be
sure that when we add ranges, we never allow the same Raft ID twice.
In order to decide which one to delete, we'll need to keep track of
a monotonically increasing sequence number for each range on creation.
If two or more ranges exist for the same raft ID, we'll take the most
recent. Ben suggests using the commit log index of the membership
change command.