[roll] Roll fuchsia [driver_runtime] Fix reset flakes.

Occasionally we would get flakes where Dispatcher::Reset() would fail
due to the drivers_ map not being empty. This is as it's legal to
call Dispatcher::Destroy() from the dispatcher shutdown callback,
and in that case we did not remove the Driver from the drivers_ map
until the runtime function (NotifyShutdown) handling the callback had
returned.

Instead the Driver object is now a RefPtr, which allows us to remove
it from the drivers_ map immediately once all dispatchers are destroyed.
The NotifyShutdown function also holds their only copy of the RefPtr.

This means when notifying for all driver destroyed, we need to track and
ensure all such NotifyShutdown functions have returned.

We also track how many threads are currently calling dispatcher shutdown
observer handlers for each driver, so we can ensure we only call the
driver shutdown observer once all these handlers have returned.
This should fix the ConcurrentDispatcherDestroy test.

Multiply: fuchsia-pkg://fuchsia.com/driver_runtime_tests#meta/driver_runtime_tests.cm : 50

Original-Bug: 127698, 120214

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/865485
Original-Revision: 2f58de1c23f79339f1085b11e0205db41beeebae
GitOrigin-RevId: 37999c406b0be8e16e53146ea6c4dbbcf2e834aa
Change-Id: Iab44544bac139455cc1b85c47791b4259a6ed5fd