• Added Scheduler.wait_and_close() to allow a grace period for tasks to complete
  before closing the scheduler.
• Added Scheduler.shield() as an alternative to asyncio.shield() which tracks
  the shielded task, thus ensuring shielded tasks are given time to complete on application
  shutdown (when used with Scheduler.wait_and_close()).
• Added support for async with syntax which will automatically call
  Scheduler.wait_and_close() when exiting the context.