When receiving a response to a request send our earlier, it is in
some cases essential, to know something about who send the reponse.
This is needed for example when sending requests to multicast
addresses to be able to react to different incoming responses.
* net/gcoap: fixed request state when getting a resp
The request state should be set to GCOAP_MEMO_RESP before calling
the response handler in case we actually go a response...
* net/gcoap: make test for 0.00 code more efficient
This optimization saves the computation of the code and compares
the raw field to `0`.
Also changed semantics for storage of message ID, from last ID to next ID.
This change provides idiomatic use of the return value of the atomic
function, without other side effects on gcoap.
