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#40234