1) Modify SessionRecord to store a list of "previous" sessions
in addition to the current active session. Previous sessions
can be used for receiving messages, but not for sending
messages.
2) When a possible "simultaneous initiate" is detected, push the
current session onto the "previous session" stack instead of
clearing it and starting over.
3) Additionally, mark the new session created on a received
possible "simultaneous initiate" as stale for sending. The
next outgoing message would trigger a full prekey refresh.
4) Work to do: outgoing messages on the SMS transport should
probably not use the existing session if it's marked stale
for sending. These messages need to fail and notify the user,
similar to how we'll handle SMS fallback to push users before
a prekey session is created.
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.
