Prepend value keys with protocol codec once it is defined

[ischasny]

The DHT specification for double hashing requires value keys to start with a codec. Add codec to the DHStore once it materialises. Things to consider:

• The codec will exist only for encryption function rotation purpose;
• We don't necessarily have to store it with each record - appending in realtime would work too.

[masih]

We probably should add the coded in go-indexer-core and validate here?

[ischasny]

Not sure yet where would be the best place to have it... The codec itself would be defined in go-multicodec, but where do we exactly add it is up for debate.