* Attempt canceling sessions by the client
In some circumstances (like a Ctrl-C), the client knows it's shutting
down and the TOFU token will be lost. In this case it makes sense to
cancel the session rather than have it time out. This introduces a
command enum to the handshake message which can be used for the client
to send commands to the worker -- in this case the only specified
command is to cancel the session. It doesn't occur if the session is
already timed out or if we can't authorize connections as this generally
means the session is already invalid.
Jeff Mitchell
Todd Knight