substitute-binary: Pipeline HTTP requests instead of using threads.

* guix/scripts/substitute-binary.scm (fetch-narinfo, %lookup-threads,
  n-par-map*): Remove.
  (narinfo-cache-file, cached-narinfo, cache-narinfo!, narinfo-request,
  http-multiple-get, read-to-eof, fetch-narinfos, lookup-narinfos,
  narinfo-from-file): New procedures.
  (lookup-narinfo): Rewrite in terms of 'lookup-narinfos'.
  (guix-substitute-binary): Use 'lookup-narinfos' instead of
  'lookup-narinfo'.