[master] (workaround for lettuce) always use asynchronous UDP server objects.

this is a simplest fix to the lettuce resolver test failure.  note: this will
make the auth server slower.  a more complete fix will be done in #1784.
okayed by muks on jabber.

src/lib/asiodns/dns_service.h

@@ -136,7 +136,7 @@ public:
     /// \throw isc::InvalidParameter if af is neither AF_INET nor AF_INET6.
     /// \throw isc::asiolink::IOError when a low-level error happens, like the
     ///     fd is not a valid descriptor or it can't be listened on.
-    void addServerUDPFromFD(int fd, int af,const UDPVersion param_flags = SYNC_);
+    void addServerUDPFromFD(int fd, int af,const UDPVersion param_flags = ASYNC_);
 
     /// \brief Remove all servers from the service
     void clearServers();