Move all the connect() logic from TCPSocketHandler into a subclass

This way, TCPSocketHandler only deal with the message sending/receiving, not
the connect() or anything else.  This will be used for implementing servers
(because when a client is accepted, we don’t need all the connect() and dns
resolution stuff).

louloulibs/network/dns_socket_handler.hpp

@@ -40,6 +40,7 @@ public:
    * Always true, see the comment for connect()
    */
   bool is_connected() const override final;
+  bool is_connecting() const override final;
   void remove_from_poller();
 
 private: