set method and the cache file handling should be concurrent access safe

[balsa02]

the set method is not concurrent access safe, because it's possible to read the cache files while they are being rewritten by a cache update request.

try {
      await ensureDirExists(path.dirname(filename));
     // you can read these files from an other green thread at the same time
      await fs.writeFile(filename + ".data", compressedValue);
      await fs.writeFile(filename + ".meta", JSON.stringify(metadata));
 }

replace them with some atomic file system method, like rename:

  try {
      await ensureDirExists(path.dirname(filename));
      const tmpdir = await fs.mkdtemp(path.dirname(filename));
      await fs.writeFile(tmpdir + "/tmp.data", compressedValue);
      await fs.writeFile(tmpdir + "/tmp.meta", JSON.stringify(metadata));
      await fs.rename(tmpdir + "/tmp.data", filename + ".data");
      await fs.rename(tmpdir + "/tmp.meta", filename + ".meta");
      await fs.rmdir(tmpdir);
    }