o Major features:
    - Relays now process the new "NTor" circuit-level handshake requests
      with higher priority than the old "TAP" circuit-level handshake
      requests. We still process some TAP requests to not totally starve
      0.2.3 clients when NTor becomes popular. A new consensus parameter
      "NumNTorsPerTAP" lets us tune the balance later if we need to.
      Implements ticket 9574.