Fix repeated key misses slowing down Version Controlled datasets

[AbhinavTuli]

🚀 🚀 Pull Request

Checklist:

• My code follows the style guidelines of this project and the Contributing document
• I have commented my code, particularly in hard-to-understand areas
• I have kept the coverage-rate up
• I have performed a self-review of my own code and resolved any problems
• I have checked to ensure there aren't any other open Pull Requests for the same change
• I have described and made corresponding changes to the relevant documentation
• New and existing unit tests pass locally with my changes

Changes

Closes #1456
New commits were created without a CommitChunkSet (until any new data was added), leading to repeated key misses that slow down iteration. If any data was added to the commit, this wasn't a problem.

This PR changes the logic to create a new empty CommitChunkSet every time a new commit/branch is created, to avoid these repeated key misses. It also tries to fix existing datasets that don't have a CommitChunkSet in any of their commits by adding it (if write access is available).

[hoshimura]

@AbhinavTuli it works! hope it gets released soon 👯