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.
The 'fixed32' type is a little more efficient here since we do not benefit from varint encoding. Or we could use the 'bytes' type, which would be more semantically correct and potentially allow us to switch hash functions if we want something stronger than crc32.