substitute: Keep the initial connection alive.

The connection used to fetch /nix-cache-info is now reused for the
subsequent narinfo requests.

* guix/scripts/substitute.scm (download-cache-info)[download]: Remove.
[uri, read-cache-info]: New variables.
Rewrite in terms of 'http-fetch' instead of 'fetch'.  Return an open
port in addition to a <cache-info>.
* guix/scripts/substitute.scm (http-multiple-get): Add #:port parameter
and honor it.
(fetch-narinfos)[do-fetch]: Add 'port' parameter.
Adjust to new 'download-cache-info' and 'do-fetch' signatures.