Always remove all the DNS sockets on an c-ares event

Because c-ares may close one of its socket, even if it’s not the one that
has the event. Otherwise we may not know when a socket has been removed from
our poller (automatically, when close()ed) and this leads to bugs.

louloulibs/network/dns_handler.hpp

@@ -40,6 +40,7 @@ public:
    * and library.
    */
   void destroy();
+  void remove_all_sockets_from_poller();
   ares_channel& get_channel();
 
   static DNSHandler instance;