wip-http-pipelining needs testing!

[Ludovic Courtès]

Last month Mark installed Nginx on Hydra, with proper caching of
narinfos.  After some debugging, wip-http-pipelining is now in a good
shape: it has performance comparable to what we currently have, *and*
it’s single-threaded and shows a progress report while downloading
substitute info (aka. narinfo.)

I would very much welcome further testing.  To do that, you need to run
the daemon like this (you need both the daemon and ‘guix
substitute-binary’ from this branch):

  sudo ./pre-inst-env guix-daemon --build-users-group=whatever ...

Then just use the guix commands with substitutes enabled.  To force an
update, you can clear the local cache with a command like:

  sudo rm -rf /var/guix/substitute-binary/cache/

See <http://lists.gnu.org/archive/html/guix-devel/2014-06/msg00069.html>
for some background.

Thanks,
Ludo’.