HTTP/2: per-iteration stream handling limit. To ensure that attempts to flood servers with many streams are detected early, a limit of no more than 2 * max_concurrent_streams new streams per one event loop iteration was introduced. This limit is applied even if max_concurrent_streams is not yet reached - for example, if corresponding streams are handled synchronously or reset. Further, refused streams are now limited to maximum of max_concurrent_streams and 100, similarly to priority_limit initial value, providing some tolerance to clients trying to open several streams at the connection start, yet low tolerance to flooding attempts.