function concurrentMapKeys (input, concurrency, asyncFn) {
  const keys = Object.keys(input);
  const values = concurrentMap(keys, concurrency, async function (key) {
    return await asyncFn(input[key]);
  });
  const ret = {};
  for (let i = 0; i < keys.length; i++) {
    ret[keys[i]] = values[i];
  }
  return ret;
}
@param {Object<string,any>} input
@param {number} [concurrency=0]
@return {Object<string,Promise>}