Instead of storing byte strings manually and manually varint encoding
integer values for incrementable keys, we now marshal and unmarshal the
Value protobuf directly. This allows us to Get incrementable keys and
to scan them. It also allows us to correctly discern an error incrementing
a key which is not a varint encoded value.

Implemented value checksums and checksum verification, although only
doing the verification now through kv/rest. Probably want to do the
same elsewhere.
Addressed all PR comments.

Added proto.Response.Verify method to verify checksums and now do verification
automatically on the responses of Get(), ConditionalPut() and Scan(). Also
now always initialize checksums on Put() and ConditionalPut().

Eventually we're going to want to fully expose the checksums to non-go
clients, but will have to evolve the KV API to include that capability.
Merge pull request #49 from cockroachdb/spencerkimball/mvcc-value-proto

Store value protobuf directly for MVCC values