[CoAP] introduced request callback for observable service
[CoAP] retransmit notifications after max-age (closes #3)
Merge branch 'coap_observe' into coap_states
[CoAP] moved observable service to coap protocol folder
[CoAP] small adaptions
Merge branch 'coap_observe' into coap_states
[CoAP] modularized code
Merge branch 'coap_observe' into coap_states
[CoAP] first concepts for high level states
[CoAP] prepared shutdown functionality