storage: add store.LastTxnEvents to improve watch #3817
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This fixes the wrong range statistics when using watch.
This avoids heavy Range call to watch events, and kills related TODO.
This makes the code cleaner.
This keeps the original logic that only returns last event on one key in one txn, but it doesn't need heavy ongoingTxn anymore. I may improve the behavior of returned events in follow-ups.
LatestTxnEvents
returns events struct today, and it may return raw kv data in the future to save the cost of marshaling and unmarshaling.