function _readLoop(state) {
  if (state.closed) return;
  readNewMessages(state).then(function () {
    return (0, _util.sleep)(state.options.idb.fallbackInterval);
  }).then(function () {
    return _readLoop(state);
  });
}
if service-workers are used,
we have no 'storage'-event if they post a message,
therefore we also have to set an interval