conncheck: Connection check request and reply transports must match

Ensure that connchecks sent with UDP don't get replied with TCP or vice versa.

Attemps to fix a Coverity issue in which no matching remote_candidate gets found for a connectivity reply in conn_check_handle_inbound_stun() (apparently due to transport mismatch), yet priv_map_reply_to_conn_check_request() still successfully matches it with a previous request.