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) At registration time, a client generates a random ID and
transmits to the the server.
2) The server provides that registration ID to any client
that requests a prekey.
3) Clients include that registration ID in any
PreKeyWhisperMessage.
4) Clients include that registration ID in their sendMessage
API call to the server.
5) The server verifies that the registration ID included in
an API call is the same as the current registration ID
for the destination device. Otherwise, it notifies the
sender that their session is stale.
