gcoap_dns: Add DNS cache support
gcoap_dns: Add DNS cache support

jasonatran/RIOT,ant9000/RIOT,kaspar030/RIOT,miri64/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,miri64/RIOT,miri64/RIOT,kYc0o/RIOT,kaspar030/RIOT,ant9000/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,kYc0o/RIOT,kYc0o/RIOT,kYc0o/RIOT,miri64/RIOT,kaspar030/RIOT,ant9000/RIOT,ant9000/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT