refactor(storage): limit use of hummock version safe epoch

[wenym1]

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What's changed and what's your intention?

As we are supporting partial checkpoint, we will maintain per table safe epoch, and the global safe epoch will be deprecated. In this PR, the scope of the global safe epoch will be narrowed down to the safe epoch of visible tables, and we will avoid using this global safe epoch in our best efforts.

The visibility of safe_epoch field in HummockVersion gets private now. Anyone that wants to access the field should explicitly call visible_table_safe_epoch method.

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[zwang28]

LGTM