The salt needs to be returned anyway, so we don't have to add another
key for the sender to recognize which request an answer is answering, so
all that infrastructure is useless. :(

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

This patch includes HMAC authenticated json messages to the serializer.
The new interface works on any json-encodable data type, and can sign it
with a private key and an optional salt. The same private key must be
used upon message loading to verify the message.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>

Reviewed-by: iustinp