Before:

  $ command time nix-prefetch-url https://cdn.kernel.org/pub/linux/kernel/v4.x/linux-4.17.6.tar.xz
  1.19user 1.02system 0:41.96elapsed 5%CPU (0avgtext+0avgdata 182720maxresident)k

After:

  1.38user 1.05system 0:39.73elapsed 6%CPU (0avgtext+0avgdata 16204maxresident)k

Note however that addToStore() can still take a lot of memory
(e.g. RemoteStore::addToStore() is constant space, but
LocalStore::addToStore() isn't; that's fixed by
https://github.com/edolstra/nix/commit/c94b4fc7ee0c7b322a5f3c7ee784063b47a11d98
though).

Fixes #1400.