[tor-relays] replace BandwidthRate with "traffic control" on busy routers

Routers running at or near a BandwidthRate setting offer terrible latency and induce connection time-outs. Refuse all DIR-port requests with "HTTP/1.0 503 Directory busy, try again later ". 'tc' ingress filter is dramatically better for rate-limiting. For case and 'tc' example see final post https://lists.torproject.org/pipermail/tor-relays/2015-October/007901.html initial post https://lists.torproject.org/pipermail/tor-relays/2015-October/007863.html