Saving should report progress

[bogovicj]

It would be nice to have a way for external tools to listen to progress as

N5Utils.save, saveRegion, etc write blocks.

Perhaps by passing a Listener to the save methods that each job can increment when a block is written.

@cmhulbert @axtimwalde

[axtimwalde]

After sleeping over this, I would first try to do this from outside because adding listeners clutters the API with things the most applications don't need. It looks like we can get meaningful feedback from the ThreadPoolExecutor, i.e. https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ThreadPoolExecutor.html#getTaskCount-- and https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ThreadPoolExecutor.html#getCompletedTaskCount--

[bogovicj]

see also saalfeldlab/n5-ij@25a3816