osmo-gsm-tester: poll the python-smpplib socket using non-busy approach
In esme.py, we currently register a poll handler (called every few ms). In that poll handler, we call python-smppllib's client.poll(), which calls non-block select to process all messages.
See https://github.com/podshumok/python-smpplib/commit/d27f070ddd99703630583a6ccebca86add593dd0 for the commit which added the APIs we use.
We should instead get the socket fd (client._socket) and register the FD to the event_loop (add a new API there (un)register_fd() or similar), which internally is going to register it with glib, so it polls it together with all other stuff.