class DriverShutdown

Asynchronously shuts down all dispatchers owned by |driver|.

|shutdown_handler| will be notified once shutdown completes. This is guaranteed to be

after all the dispatcher's shutdown observers have been called, and will be running

on the thread of the final dispatcher which has been shutdown.

While a driver is shutting down, no new dispatchers can be created by the driver.

If this succeeds, you must keep the |shutdown_handler| object alive until the

|shutdown_handler| is notified once the last dispatcher completes shutting down.

# Thread safety

This class is thread-safe.

# Errors

ZX_ERR_INVALID_ARGS: No driver matching |driver| was found.

ZX_ERR_BAD_STATE: A driver shutdown observer was already registered.

If |Begin| was called successfully, you must keep this alive until the |Handler|

has been called.

This may be destructed from any thread.