o Critical bugfixes:
    - Distinguish downloading an authority certificate by identity digest from
      downloading one by identity digest/signing key digest pair; formerly we
      always request them only by identity digest and get the newest one even
      when we wanted one with a different signing key.  Then we would complain
      about being given a certificate we already had, and never get the one we
      really wanted.  Now we use the "fp-sk/" resource as well as the "fp/"
      resource to request the one we want.  Fixes bug 5595.