Write Incoming MSG into namespaced blob

* Any payload MUST arrive already encrypted to the endpoint of the Incoming

Box. Soledad, at v1, will not add any encryption to the payloads.

* The knowledge to decrypt a given payload, at v1, is shared by both the Trusted

App that delivered the payload into the user Incoming Box (MX in this case),

and the domain-specific application that processes the incoming message on the

client side (incoming service in bitmask mail, in this case).