SimpleWorkflow service support for long polling - EUCA-9621

A stateful polled notifications service is added for workflow task
notifications. The simple workflow service sends notifications to the
polled notifications service when pending tasks are queued. When the
simple workflow service is polled for tasks it in turn polls the
notifications service.

When the polled notifications service is disabled all pollers are
notified and on their next poll will be directed to the currently
enabled polled notifications service.

EUCA-9621