Old session not expired prior to registration of new session when a single client connects

[kuujo]

When a single client is connected to the cluster and crashes, its session will not be expired until a new session is registered. This is because session expiration is dependent on keep-alives or commands being submitted by a client to progress time in the state machine. However, when a new client connects, the client's registration event occurs prior to the old session being expired. The session expiration event should occur prior to registration of the new session in the state machine.