Updated openstack/openstack
Project: openstack/oslo.messaging  39e4a0bcc40a17e42012b64e19e4abb75509d9b1

Add a thread + futures executor based executor

The concurrent.futures module is one of the ways
that async activities can be done in the future,
so we should try to work on getting to that future
by using more futures. To enable this (as well as
to enable getting off eventlet), add a thread pool
based executor which will process incoming messages
using the pool.

Also begins adding according docs as well for the
different types of executors that are available.

Change-Id: I1482fd70abbf69f4e2994597c5e95d91fecb815e