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/libutil/util.cc

@@ -1215,4 +1215,15 @@ string base64Decode(const string & s)
 }
 
 
+void callFailure(const std::function<void(std::exception_ptr exc)> & failure)
+{
+    try {
+        failure(std::current_exception());
+    } catch (std::exception & e) {
+        printMsg(lvlError, format("uncaught exception: %s") % e.what());
+        abort();
+    }
+}
+
+
 }