[GraphBolt] Async feature fetch refactor

[mfbalin]

Description

@frozenbugs can you take a look at the overall API and let me know how it looks? I will separate into small PRs later, this is a draft PR.

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
• I've leverage the tools to beautify the python and c++ code.
• The PR is complete and small, read the Google eng practice (CL equals to PR) to understand more about small PR. In DGL, we consider PRs with less than 200 lines of core code change are small (example, test and documentation could be exempted).
• All changes have test coverage
• Code is well-documented
• To the best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change
• Related issue is referred in this PR
• If the PR is for a new model/paper, I've updated the example index here.

Changes

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[mfbalin]

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[mfbalin]

@frozenbugs, I have merged the contents of this PR in 10 other PRs. If you review them and let me know suggestions and improvements, I can address the reviews in this PR and we can merge this one too.

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[mfbalin]

@frozenbugs I have address your reviews in this initial PR. Further review comments can be addressed in this PR too.

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