optional encryption

[will-rowe]

Initial thought is to encrypt record fields and set a tag to tell retrievers the record needs decrypting before use.

[will-rowe]

I've added optional encryption of Records via 430aa54

At the moment, it's using symmetric key encryption to encrypt and decrypt individual Record fields prior to protobuf/json serialisation. This restricted to the UUID for now until I figure out if this is the best approach and a useful feature.