feat(storage): decouple spill task from epoch

[wenym1]

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What's changed and what's your intention?

After #17635.

Introduce a new spill policy that does not depend on epoch order. We will spill the epoch that has the most spillable data. For spillable data, it means the data is the oldest un-spilled data in the local instance. When some data is spilled, some data that was not previously spillable may become spillable because some data older than it gets spilled. Therefore, when some data are spilled, we will update the spillable data information so that we can spill more data in batch in later round of spill.

A unit test is added, and the previously test utils are moved to a separate test_utils.rs file.

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[hzxa21]

LGTM