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.
We could check the checksum here, but the idea behind this particular checksum is to verify at all the way at the requesting client. Keep in mind that fetching the block(s) from disk which contain the requested value involves checksum verification at the level of RocksDB, so this check would be redundant and crc32 while fast, isn't free.