Stop exposing the greenthread abstraction in compute.manager

[Joshua Harlow]

In order to make it easier to someday (or maybe not someday) move off of greenlet/eventlet it would be nice to not always expose things like greenthread.sleep(0) in code, but instead put an abstraction layer up that performs this action instead.

For example:

for bw_ctr in bw_counters:
    # Allow switching of greenthreads between queries.
    greenthread.sleep(0)
    # Do work...

Instead this could be:

for bw_ctr in task_switcher_iter(bw_counters):
    # Do work...

Notice that we can now make the 'task_switcher_iter' do the greenthread.sleep
or not do it at all, thus making the code cleaner and making it easier to drop
eventlet if this ever becomes a issue (aka with python3 lacking eventlet support)

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/25751

[Matt Riedemann (mriedem)]

The patch was abandoned with a lot of comments from Chris Behrens, is this worth pursuing?