[coap] Improve behaviour of CoAP multicast requests & responses (#3967)
[coap] Improve behaviour of CoAP multicast requests & responses (#3967)

This commit prevents the CoAP stack from sending automatic 'Not
Found' responses to multicast CoAP requests, as suggested in RFC7252
8.2:

   When a server is aware that a request arrived via multicast, the
   server MAY always ignore the request, in particular if it doesn't
   have anything useful to respond (e.g., if it only has an empty
   payload or an error response).

This commit also allows CoAP requests that are sent to multicast
addresses to receive multiple responses, which will all be processed
by the same handler callback (For instance, a multicast GET to
discover network resources).
srickardti/openthread,chshu/openthread,turon/openthread,openthread/openthread,srickardti/openthread,turon/openthread,jwhui/openthread,jwhui/openthread,abtink/openthread,lanyuwen/openthread,librasungirl/openthread,librasungirl/openthread,lanyuwen/openthread,chshu/openthread,chshu/openthread,openthread/openthread,lanyuwen/openthread,srickardti/openthread,turon/openthread,bukepo/openthread,abtink/openthread,bukepo/openthread,abtink/openthread,jwhui/openthread,librasungirl/openthread,turon/openthread,lanyuwen/openthread,jwhui/openthread,abtink/openthread,lanyuwen/openthread,openthread/openthread,librasungirl/openthread,bukepo/openthread,openthread/openthread,chshu/openthread,chshu/openthread,chshu/openthread,lanyuwen/openthread,bukepo/openthread,srickardti/openthread