net: lib: coap_client: Don't clear internal structures on response

When response is received and handled, don't just clear the structure
but instead mark it as ongoing=false.
So if we later on receive a duplicate response for it, we can still
respond with Ack or Rst.

This is achieved by using release_internal_request() when we don't
expect any response for it and reset_internal_request() when we really
fill up a new request.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>