Fix crashing in dns udp relay and handle more then 1 query

While testing relayd as a dns relay we encountered the issue that relayd immediately crashes at the first request. We traced this to an incorrect sc_timeout which we replaced with the sc_timeout used everywhere else.

After solving this issue we got a reply over UDP, but after the first reply the relayd would stop serving any other requests. Looking at the TCP handler we concluded the addition of an event_add in the UDP server. At a first glance everything seems to work now, though this code would require the additional review of someone with a bit more expertise concerning relayd and libevent.