Check on race conditions about datafs HOT 3 OPEN

@jgerardsimcock putting a pin in this - there's a way to lock fs objects across threads even outside of the core methods. Maybe there is something we could be doing to prevent simultaneous access across multi-step operations such as cache + read, for example.

http://pyfilesystem.readthedocs.io/en/latest/base.html

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I agree that once refactored we definitely must be able to handle all of these. However, I don't think we should use last-accessed for any of our systems. I think we should use 'all' or, if space constraints are a problem, 'latest'.

Furthermore, (3) seems to me to be a pathological case and, while it's important that DataFS behaves correctly under these circumstances, I don't see any problem if there is a significant performance penalty for behavior consistent with (3). Does that seem right to you?

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Agreed that performance penalties on (3) are fine. But it's not that pathological: It is fairly common for us to have two different versions of the same sector running at the same time. E.g., we are producing a set of results, and then find a way to improve one of the underlying datasets. Rather than kill the first one (since we might decide to not use the improvement in the end), we just spin up another set of runs with the other dataset.

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