Move BinaryCacheStore / LocalBinaryCacheStore from Hydra

So you can now do:

  $ NIX_REMOTE=file:///tmp/binary-cache nix-store -qR /nix/store/...

src/libstore/local-binary-cache-store.hh

@@ -0,0 +1,31 @@
+#pragma once
+
+#include "binary-cache-store.hh"
+
+namespace nix {
+
+class LocalBinaryCacheStore : public BinaryCacheStore
+{
+private:
+
+    Path binaryCacheDir;
+
+public:
+
+    LocalBinaryCacheStore(std::shared_ptr<Store> localStore,
+        const Path & secretKeyFile, const Path & publicKeyFile,
+        const Path & binaryCacheDir);
+
+    void init() override;
+
+protected:
+
+    bool fileExists(const std::string & path) override;
+
+    void upsertFile(const std::string & path, const std::string & data) override;
+
+    std::string getFile(const std::string & path) override;
+
+};
+
+}