publish: Do not load archive content in memory.

Previously, before replying to a /nar/* request, 'guix publish' would first
build up the whole nar into memory (as a consequence of
<http://bugs.gnu.org/21093>), which obviously doesn't scale.

* guix/scripts/publish.scm (render-nar): Return STORE-PATH instead of a
  procedure that calls 'write-file'.
  (sans-content-length): New procedure.
  (http-write): For 'x-nix-archive', don't call '%http-write'.  Instead, call
  'write-file' right from here, using BODY as the file name.

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