fix(storage): fix leaving put key after delete keys are dropped cause inconsistent

[Little-Wallace]

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What's changed and what's your intention?

fix #15057 (comment)

When we put some keys and then delete them in one epoch, if hummock enable "spill memtable", and then compact them in the last level of LSM tree, compactor may lost DELETE keys when and only when the safe watermark keeps same with just flushed epoch. We used to assume that the epoch of current keys must be larger than the follower keys, but it is wrong since we start using pure epoch to decide whether it shall be dropped.

So compactor drops the delete keys and left the put key because they share the same pure_epoch and it exactly equals task_config.watermark.

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