tor/changes/ticket24902

  o Major features (denial of service mitigation):
    - Give relays some defenses against the recent network overload. We start
      with three defenses (default parameters in parentheses). First: if a
      single client address makes too many concurrent connections (>100), hang
      up on further connections. Second: if a single client address makes
      circuits too quickly (more than 3 per second, with an allowed burst of
      90) while also having too many connections open (3), refuse new create
      cells for the next while (1-2 hours). Third: if a client asks to
      establish a rendezvous point to you directly, ignore the request. These
      defenses can be manually controlled by new torrc options, but relays
      will also take guidance from consensus parameters, so there's no need to
      configure anything manually. Implements ticket 24902.