add store access interface to ranges

address feedback @andybons
update git workflow
Merge pull request #45 from tobstar87/rangeaccess

add store access interface to ranges
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.
Remove joint consensus (ProposedMembers) support.

This was present in the original raft paper but was removed from the final
thesis because simpler protections suffice as long as only one node is added
or removed at a time.

Use the term "observer" consistently for non-voting members.
Merge pull request #46 from cockroachdb/spencerkimball/rename-internal-range-scan

Rename InternalRangeScan to InternalSnapshotCopy.
Merge branch 'raft'

* raft:
  Remove joint consensus (ProposedMembers) support.
add weighted reservoir sampling (AE-S)

address feedback from @spencerkimball

final commit before merge