o Minor features (directory client):
    - When downloading server- or microdescriptors from a directory server,
      we no longer launch multiple simultaneous requests to the same server.
      This reduces load on the directory servers, especially when directory
      guards are in use.  Closes ticket 9969.
    - When downloading server- or microdescriptors over a tunneled
      connection, do not limit the length of our request to what the Squid
      proxy is willing to handle.  Part of ticket 9969.