net: coap_client: Stop all socket activities when cancelling requests

Calling coap_client_cancel_requests() clears the internal request
context only for active requests (i. e. not replied yet). However,
if there are any pending request context monitoring ACK duplicates,
those would still make the corresponding client socket being monitored
by poll(). In result, when application closes the socket, the polling
thread will throw POLLNVAL error for the socket.

Fix this, by resetting all request contexts unconditionally. The request
callback will only be called for the active requests.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>