Use list instead of skiplist to store data if `TTLType` is `Latest`

OpenMLDB use double-layer skiplist to store data in memory storage engine currently. Skiplist will take some extra memory in header and data nodes. As `Lastest` table has little records in second layer skiplist, using list will not have a noticeable impact on performance.

Look at the memory storage engine's data structures, focusing on where the double-layer skiplist is implemented for tables with TTLType 'Latest'. Identify the relevant files (likely in storage engine modules) and understand how to replace the skiplist with a list without affecting performance. Check existing tests for the storage engine to ensure the change works correctly.

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