MXS-2196: Remove DCB from Listener

Replaced the DCB with a single file descriptor that the listener listens
on and which is added to all of the workers. The Listener also extends the
MXB_POLL_DATA which allows it to handle epoll events.

Moved the code that creates the listening socket into listener.cc where it
belongs and did a minor cleanup of it.

server/core/service.cc

@@ -323,7 +323,7 @@ bool service_isvalid(Service* service)
 static int serviceStartPort(Service* service, const SListener& port)
 {
     mxb_assert(service && service->router && service->router_instance);
-    return port->listen(port);
+    return port->listen();
 }
 
 /**