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Waits until a new checkpoint file is found.

checkpoint_dir The directory in which checkpoints are saved.
last_checkpoint The last checkpoint path used or None if we're expecting a checkpoint for the first time.
seconds_to_sleep The number of seconds to sleep for before looking for a new checkpoint.
timeout The maximum number of seconds to wait. If left as None, then the process will wait indefinitely.

a new checkpoint path, or None if the timeout was reached.