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.
Actually, I looked it up--crc32 checksum of zero is just as likely as any other, so we can't rely on 0 meaning no checksum; it's perfectly valid. That's why I changed this to be nullable.