Sun Dec 3 00:58:21 EST 2017

nix store + btrfs snapshots

- lxc debian container creation
- btrfs snapshots
- nix build system

Use nix store to store references to btrfs snapshots stored on a
separate host.  After running nix-gc, delete all snapshots that are no
longer referenced.

The main point of insight is that it is only necessary to have one
central garbage-collected store.  On top of this it is straightforward
to add external objects.