message: refactor(service-worker): notify clients about updates in parallel
  Previously, clients were notified about updates sequentially. This
  wasn't necessary.
  This commit changes the `Driver#notifyClientsAboutUpdate()` method to
  notify the clients in parallel (by switching from `Array#reduce()` to
  `Array#map()` and `Promise.all()`).
  This also aligns the `notifyClientsAboutUpdate()` method with the
  `notifyClientsAboutUnrecoverableState()` method.
  PR Close
diff: @@ -1037,9 +1037,7 @@ export class Driver implements Debuggable, UpdateSource {
  const clients = await this.scope.clients.matchAll();
  - await clients.reduce(async (previous, client) => {
  - await previous;
  -
  + await Promise.all(clients.map(async client => {
  // Firstly, determine which version this client is on.
  const version = this.clientVersionMap.get(client.id);
  if (version === undefined) {
  @@ -1062,7 +1060,7 @@ export class Driver implements Debuggable, UpdateSource {
  };
  client.postMessage(notice);
  - }, Promise.resolve());
  + }));
  }
  async broadcast(msg: Object): Promise<void> {
  