fix(meta): record delta for new compaction group

[zwang28]

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What's changed and what's your intention?

There's an optimization in time travel metadata: it will drop any delta log that doesn't add any SSTs. Because previously a new delta log from commit_epoch always belongs to either compaction group 2 or 3, which must exist.

However, during snapshot backfill it's possible that a delta log adds a new compaction group without adding any SSTs, e.g. it includes GroupConstruct but doesn't include any NewL0SubLevel.
This PR fixes the bug by enforcing a new version for time travel for the above new case, similar to how it is handled for delta logs generated by compaction.

This PR also removes useless time_travel_toggle_check, since time travel is always enabled.

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[Li0k]

LGTM, thanks for the quick fix