Broadcast Bounce Mitigation

[jrconlin]

Broadcast wants to optimize how we distribute notifications to clients.

Hashing out the use cases and requirements here:

https://docs.google.com/document/d/1GjpZSxwhGzn4tdYI_72e17riBVU0xy4nVjF5qTEOLCI/edit?ts=5f55f5f0#