Introduce registration-time ID for detecting stale sessions.

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.