Implement nix-copy-closure --from via nix-store --serve
Remove obsolete SSH master connection code
Fix NIX_SSHOPTS

Closes #302.
nix-copy-closure: Implement --gzip via ssh's -C flag
nix-copy-closure: Drop --bzip2, --xz, --show-progress

These are too difficult to implement via nix-store --serve.

‘--show-progress’ could be re-implemented fairly easily via a
sink/source wrapper class.