For asynchronous programming, a timeout parameter is required on the listener
to allow to stop it at exit. poll() returns None on timeout.
It plan to use it in my new asyncio (Trollius) executor:
https://review.openstack.org/#/c/70983/
See also the related blueprint for the rationale:
https://wiki.openstack.org/wiki/Oslo/blueprints/asyncio
Change-Id: I918ae3c267743a0eaed1d6a210c79fb4a0eb8733