BinaryCacheStore: Support local caching of NARs

This speeds up commands like "nix cat-store". For example:

  $ time nix cat-store --store https://cache.nixos.org?local-nar-cache=/tmp/nar-cache /nix/store/i60yncmq6w9dyv37zd2k454g0fkl3arl-systemd-234/etc/udev/udev.conf
  real    0m4.336s

  $ time nix cat-store --store https://cache.nixos.org?local-nar-cache=/tmp/nar-cache /nix/store/i60yncmq6w9dyv37zd2k454g0fkl3arl-systemd-234/etc/udev/udev.conf
  real    0m0.045s

The primary motivation is to allow hydra-server to serve files from S3
binary caches. Previously Hydra had a hack to do "nix-store -r
<path>", but that fetches the entire closure so is prohibitively
expensive.

There is no garbage collection of the NAR cache yet. Also, the entire
NAR is read when accessing a single member file. We could generate the
NAR listing to provide random access.

Note: the NAR cache is indexed by the store path hash, not the content
hash, so NAR caches should not be shared between binary caches, unless
you're sure that all your builds are binary-reproducible.

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