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

[dl239]

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.

[longxu0509]

Please assign this task to me.