sock/async: introduce function to retrieve session of current DTLS event
sock/async: introduce function to retrieve session of current DTLS event

ant9000/RIOT,kYc0o/RIOT,OlegHahm/RIOT,miri64/RIOT,kYc0o/RIOT,miri64/RIOT,kYc0o/RIOT,kaspar030/RIOT,kYc0o/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,miri64/RIOT,OlegHahm/RIOT,jasonatran/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,kYc0o/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,ant9000/RIOT,ant9000/RIOT,authmillenon/RIOT,ant9000/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kaspar030/RIOT,OTAkeys/RIOT,jasonatran/RIOT,authmillenon/RIOT,miri64/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,authmillenon/RIOT,authmillenon/RIOT,jasonatran/RIOT,jasonatran/RIOT,ant9000/RIOT,OlegHahm/RIOT,kaspar030/RIOT,miri64/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kaspar030/RIOT