NSFS | versioning | change POSIX to use file descriptors and linkat instead of paths and current safe_move

[nadavMiz]

Environment info

• NooBaa Version: VERSION
• Platform: Kubernetes 1.14.1 | minikube 1.1.1 | OpenShift 4.1 | other: specify

Actual behavior

1.currently in posix we work with paths in order to move files. paths that might change by different threads / processes. to account for that we use safe_move_posix function that validates the source and destination files are the ones expected. but safe_move is not atomic and might not work properly in certain situations. also when safe_link does prevent the wrong file move, it causes a retry that could have been avoided by using file descriptors.
we should change it to work with FD and linkat similerly to how it works in POSIX

Expected behavior

Steps to reproduce

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