Make computeFSClosure() single-threaded again

The fact that queryPathInfo() is synchronous meant that we needed a
thread for every concurrent binary cache lookup, even though they end
up being handled by the same download thread. Requiring hundreds of
threads is not a good idea. So now there is an asynchronous version of
queryPathInfo() that takes a callback function to process the
result. Similarly, enqueueDownload() now takes a callback rather than
returning a future.

Thus, a command like

  nix path-info --store https://cache.nixos.org/ -r /nix/store/slljrzwmpygy1daay14kjszsr9xix063-nixos-16.09beta231.dccf8c5

that returns 4941 paths now takes 1.87s using only 2 threads (the main
thread and the downloader thread). (This is with a prewarmed
CloudFront.)

src/libstore/store-api.hh

@@ -319,9 +319,16 @@ public:
        the name part of the store path. */
     ref<const ValidPathInfo> queryPathInfo(const Path & path);
 
+    /* Asynchronous version of queryPathInfo(). */
+    void queryPathInfo(const Path & path,
+        std::function<void(ref<ValidPathInfo>)> success,
+        std::function<void(std::exception_ptr exc)> failure);
+
 protected:
 
-    virtual std::shared_ptr<ValidPathInfo> queryPathInfoUncached(const Path & path) = 0;
+    virtual void queryPathInfoUncached(const Path & path,
+        std::function<void(std::shared_ptr<ValidPathInfo>)> success,
+        std::function<void(std::exception_ptr exc)> failure) = 0;
 
 public: