rate-limiting: propagate back-pressure from queue as HTTP 429's

[yaauie]

Adds a proactive handler that rejects new requests with HTTP 429's when the queue has been blocking for more than 10 consecutive seconds, allowing back- pressure to propagate in advance of filling up the connection backlog queue.

While I'd like to take the time to break out the supplied ExecutionObserver into a separate library, I think it is better to roll with what we have than to hold back.