net: coap_client: Stop lifetime on piggybacked Ack

When we receive piggybacked Ack, it means that we have
received all information for this request.
There is no need to track exchange lifetime anymore.

Once we reset the internal request, it is free to be used
for the application. So if we only have few requests allocated,
it would be slow to send those as get_free_request() only
gives request structures that don't have a lifetime left.

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