o Minor bugfixes (directory authority):
    - When a directory authority rejects a descriptor or extrainfo with
      a given digest, mark that digest as undownloadable, so that we
      do not attempt to download it again over and over. We previously
      tried to avoid downloading such descriptors by other means, but
      we didn't notice if we accidentally downloaded one anyway. This
      behavior became problematic in 0.2.7.2-alpha, when authorities
      began pinning Ed25519 keys. Fixes ticket
      22349; bugfix on 0.2.1.19-alpha.