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

[Dhalgren Tor]

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