minor cleanups on proposal 104

  well-behaved tools would need to download short-form descriptors too (as

  these would be the only ones indexed), and hence get redundant info. Badly

  behaved tools would download only long-form descriptors, and expose

  themselves to partitioning attacks and that like.)

  themselves to partitioning attacks.)

Other disposable fields:

          extra-info documents, or an authority that handles extra-info

          documents.

     (Since implementations before 0.1.2.5-alpha required that "opt" keyword

     precede any unrecognized entry with "opt", these keys MUST be preceded

     (Since implementations before 0.1.2.5-alpha required that the "opt"

     keyword precede any unrecognized entry, these keys MUST be preceded

     with "opt" until 0.1.2.5-alpha is obsolete.)

  3. New communications rules

     Servers SHOULD generate and upload an extra-info document after each

     Servers SHOULD generate and upload one extra-info document after each

     descriptor they generate and upload; no more, no less.  Servers MUST

     upload the new descriptor before they upload the new extra-info.

     Authorities receiving an extra-info document SHOULD verify all of the

     following:

       * They have a router descriptor for some server with a matching

         nickname, IP, and identity fingerprint.

         nickname and identity fingerprint.

       * That server's identity key has been used to sign the extra-info

         document.

       * The extra-info-digest field in the router descriptor matches