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.
Computing the checksum in this way allows bytes to be shuffled between the key and value without changing the checksum. If we were using this checksum for security that would be a problem; I don't know if it matters here since we're using a weak checksum.  If you want to do something about that I suggest hashing `len(key) + key + len(value) + value`.