Explicitly close the host callback queues inside `IfrtBackend` destruction
Explicitly close the host callback queues inside `IfrtBackend` destruction

If there are host callback executions that are blocked inside `RemoteLoadedHostCallbackQueue::Pop()`, they will not be cancelled automatically unless `RemoteLoadedHostCallbackQueue::Close()` is called. This causes a deadlock since `IfrtBackend` also waits for all in-flight operations to finish.
