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.cpp

@@ -40,6 +40,11 @@ bool DNSSocketHandler::is_connected() const
   return true;
 }
 
+bool DNSSocketHandler::is_connecting() const
+{
+  return false;
+}
+
 void DNSSocketHandler::remove_from_poller()
 {
   if (this->poller->is_managing_socket(this->socket))