[roll] Roll fuchsia [vfs] Rework how ExecutionScope's shutdown works

This changes the way that ExecutionScope's shutdown feature works which
simplifies most uses.

With this new mechanism, after calling shutdown all tasks will continue
running whilst at least one is considered active. A guard is used to
mark a task as active. This will mean that some tasks run for longer
than they might have done prior to this change, but this should not have
any material difference.

Tasks that want async behaviour to take place when they are dropped
(rare) can spawn tasks from their drop functions and the executor will
delay shutdown until these async tasks have finished.

This results in a small decrease in binary size and it should take less
runtime memory too, but the main benefit is that spawning tasks is
simpler for the majority of uses.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/858276
Original-Revision: a24d776f25c78f7784080fb6e8c61be860243051
GitOrigin-RevId: 06c8c1f5d37f4faf1fae3302635a7df35b861efb
Change-Id: I0cd8f6e56afbbb3b0807e9ac6ecdc04c72482f2c