ConcurrentRequestsHttpConnectionFilter should leverage NettyConnection.onClosing() instead of ConnectionContext.onClose()

[jayv]

We used to set maxPipelinedRequests = 0 from NettyConnection.onClosing() which lead to misleading error messages. With the improved API on RequestConcurrencyController to indicate RejectedPermanently, we can minimize unnecessary failed requests on connection closure by triggering RejectedPermanently from NettyConnection.onClosing() however it’s not exposed.

AbstractStreamingHttpConnection has been modified to work around the shortcoming that ConcurrentRequestsHttpConnectionFilter has no access to onClosing(), once we make it available on NettyConnectionContext we must also update AbstractStreamingHttpConnection to leverage onClosing() again.

[idelpivnitskiy]

ConcurrentRequestsHttpConnectionFilter was removed in #948.