Rename InternalRangeScan to InternalSnapshotCopy. This eliminates some
confusion between scanning as with MVCC and doing a raw scan of engine
contents. The "snapshot copy" gives a much better idea of the proper use
and limits of this RPC.
Merge pull request #46 from cockroachdb/spencerkimball/rename-internal-range-scan

Rename InternalRangeScan to InternalSnapshotCopy.