Broadcast rate limit sleep events

[dmyers]

It would be neat to be able to easily log and identify what is being rate limited and for how long if there was a way to get the current store and deferrer instances as event data.

[sebastiandedeyne]

I'd accept a PR that uses Guzzle's event system for this.

https://docs.guzzlephp.org/en/5.3/events.html