`hard_limit` needs to support multiple rate limiting schemes

[BlinkyStitt]

Right now we only per-second rate limits to our backend rpcs (its the most common). But some providers do a rolling window over 1 minute. Others have more complex schemes that allow for bursting.

Given that we run most of our own backend servers and so don't have rate limits, this is low priority.

[BlinkyStitt]

0bc523a makes it so we can have different window sizes. we don't support rolling windows though.