Fix issue where SemaphoreSlim is released twice in RateLimitGate

[Jonnern]

CheckGuardsAsync releases the semaphore when a request is rate-limited. If this task is canceled, the caller ProcessAsync, will also release the semaphore in the finally block. After this point, the RateLimiter is no longer thread-safe.

The implementation assumes that CheckGuardsAsync cannot throw a TaskCanceledException before the semaphore has been released.

[JKorf]

Hi, I had to look for a bit but you're right, good find!
Could you also add it to the ProcessSingleAsync method?

[Jonnern]

I updated ProcessSingleAsync as well and squashed the commits

[Jonnern]

@JKorf Would it be better to return
return new CallResult(new CancellationRequestedError()); in the catch block than rethrow?

[JKorf]

Look at it now, I think you're right. If you cancel the wait then I think the exception isn't actually caught anywhere. I'll fix that with the new release