Add an optional timeout parameter to Listener.poll

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
Merge "Add an optional timeout parameter to Listener.poll"