Store value protobuf directly for MVCC values

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.
From many of the comments here, I'm thinking it might make sense to rename the Checksum field "EndToEndChecksum". The idea is this checksum is set by the client -- perhaps as far back as from a Javascript client speaking over http, and then verified by any similar endpoint. It's meant to capture corruption anywhere in the communication stack, up to and including the final network interface and even the client's own potential memory stomping bugs.

As such, there's no reliable way to include the timestamp--remember that timestamps on writes can change if part of a transactions, and for non-transactional writes, the timestamp is taken as the receiving node's wall clock time, so can't be incorporated by the original client.