Free sockets awaiting upstream DNS replies ASAP.

Once we have a good answer, close the socket so that the fd can
be reused during DNSSEC validation and we don't have to read and
discard more replies from other servers.

src/forward.c

@@ -1144,6 +1144,13 @@ void reply_query(int fd, time_t now)
     }
 
   forward->sentto = server;
+
+  /* We have a good answer, and will now validate it or return it. 
+     It may be some time before this the validation completes, but we don't need
+     any more answers, so close the socket(s) on which we were expecting
+     answers, to conserve file descriptors, and to save work reading and
+     discarding answers for other upstreams. */
+  free_rfds(&forward->rfds);
   
 #ifdef HAVE_DNSSEC
   if ((forward->sentto->flags & SERV_DO_DNSSEC) &&