Discard requests that are delayed longer than a configured threshold

The RequestThrottler ensures that no requests more than the system can handle be fed into the request processor pipeline. In the meantime, the throttler queues all incoming requests and there is nothing to instruct the clients to slow down.

This new feature will mark all requests that wait in the RequestThrottler longer that specified throttledOpWaitTime as throttled and such requests will not see any processing other than being fed down the pipeline preserving the order of all requests.

The FinalProcessor will issue an error response (new error code: ZTHROTTLEDOP) for these undigested requests. The intent is for the clients to not retry them immediately.

Also the fact that throttled requests are unprocessed will speed the entire work of the pipeline. Throttled requests are not communicated between servers and only travel thru the server they belong to.