Handle throttled events/functions

[cohlar]

It's great to be able to limit a function's rate with rateLimit, however throttled events are kind of "lost" - they do not trigger any function, and the client that sends the event is unaware of that.

It would be nice to be able to handle those throttled events. Two ideas of how that could work:

1. Add an option to rateLimit to retain throttled events in the queue and execute the function later once it's back within the rate limit
2. Add a onThrottle() handler to functions which would be called every time an event is received and the function is throttled