Bug/feature: Rate-throttling logic is global and not per-token

When I added the rate-throttle logic I did so on the assumption that the process would be making requests on behalf of a single token. I think this is still the most common case, but I didn't account for the scenario where one process might make many requests using many different tokens (think if you've written some kind of a proxy service and operates on each individual token).

The way to move forward is to make the RateThrottleClient instance local to each instance of `PlatformAPI`. This will require work in Heroics since that's what we hook into to provide this feature.