1) On the push side, this message is a flag in PushMessageContent.
Any secure message with that flag will terminate the current
sessin.
2) On the SMS side, there is an "end session" wire type and
the convention that a message with this wire type must be
secure and contain the string "TERMINATE."
1) In addition to the Recipient interface, there is now
RecipientDevice. A Recipient can have multiple corresponding
RecipientDevices. All addressing is done to a Recipient, but
crypto sessions and transport delivery are done to
RecipientDevice.
2) The Push transport handles the discovery and session setup
of additional Recipient devices.
3) Some internal rejiggering of Groups.
1) Move the attachment structures into the encrypted message body.
2) Encrypt attachments with symmetric keys transmitted in the
encryptd attachment pointer structure.
3) Correctly handle asynchronous decryption and categorization of
encrypted push messages.
TODO: Correct notification process and network/interruption
retries.
1) Add encryption support for the transport layer. This obscures
metadata from the push messaging provider.
2) Better support the direction multiple destination messages is
headed (one unique message per recipient).
