Offloading and compression

[Ludovic Courtès]

Commit 8b7af63 changes the offload so it compresses store items sent
from one build machine to another.

Initially I was blissfully hoping that using SSH-level compression (via
‘lsh -z’) would be enough, but I noticed that hydra.gnu.org was spending
most of its time transferring stuff, hence this change.

I’ve deployed the new daemon and offload hook on hydra.gnu.org, so
hopefully things will be a little faster now.

Ludo’.