conncheck: add a check to move into the ready state after a pair failed

This patch tries to move the component state from connected to ready in places where a pair may fail. Consequently, the final check done after the expiration of the idle timeout can be removed, assuming that transitions are done as soon as pairs state makes it possible.

The only place where such a situation has been observed in a real world stress test is a 401 unauthorized stun error received in priv_map_reply_to_conn_check_request(), when the conncheck contains a local and a remote candidate, both of type host, with an identical IP address and port number (two different boxes with a private network using the same subnet). In such a case, a stun request to the remote candidate will reach the local candidate instead, and will logically fail.