Difficulties terminating and resuming DTLS sessions

[pelzvieh]

DTLS session termination message from server might be lost, e.g. when the client hibernates or has a temporary lack of network connection. Situations can be seen when the client tries to resume the session after wakeup/network connection. Those packets are silently ignored by the server, as the corresponding session object is already deregistered and the DatagramSocket disconnected.