awaitAsyncCompletion method
Await for all (including future) async submissions to be completed (the async queue becomes idle for a moment).
Returns true if all submissions were completed or async processing was not started; false if shutting down (or an internal error occurred).
Use to wait until all puts by Box.putQueued have finished.
Implementation
bool awaitAsyncCompletion() {
final result = C.store_await_async_submitted(_ptr);
reachabilityFence(this);
return result;
}