Get a reference implementation of jobs/jobboard for taskflow

Currently we don't have a full (basic) implementation of the job/jobboard code. It would be nice to establish a reference implementation of this (using zookeeper or something similar) so that we can have a working example of this section of code to show others how HA work can be transferred & resumed from one entity...

Instead of exposing raw green threads, hide them. for OpenStack Compute (nova)

Currently there are places in nova which expose and use the greenthread code directly for various reasons. It would be nice to move to a more abstracted model where the greenthread implementation is only exposed in 'utils.py' and everything else uses modules/code there instead of using greenthread directly. This see...