Test effects of concurrent mutations on directory structure

[jamesbornholt]

Right now we don't have any test coverage of concurrent mutation—what happens if objects are created/mutated/deleted on the S3 side while Mountpoint is interacting with them. We need to get test coverage of these behaviors and then define our intended semantics for them.

• Concurrent object mutation while reading -- cause future reads to fail when detected
• Concurrent creation/removal of implicit directories -- create/remove foo/bar when foo already existed
• Concurrent creation of explicit directories -- create/remove foo/ when foo already existed

[sauraank]

I think fuse test for prefetching covers the case of causing failure to future reads when concurrent mutation are done while reading? Or we talking about something else here?