Add support for broadcast push notifications

[jchambers]

This is (for now!) a work-in-progress draft of support for broadcast push notifications. It turns out it's a pretty big project that involves building a client for an entirely new part of APNs: the channel management service.

When it's done, this will close #1081.

TODO:

• [x] Channel management system (#1107)
  • [x] Tests
  • [x] Docs
• [ ] Plumbing for actually sending broadcast notifications
  • [ ] Notification/response interfaces
  • [ ] Actual implementation
  • [ ] Tests
  • [ ] Docs
• [ ] New payload fields
• [ ] Update metrics interfaces
• [ ] Update copyright headers