mem table spill only uses the last 12 bits of epoch

[wcy-fdu]

This is a feature request by recursive cte, which tend to use the high 4 bits of the last 16 bits of epoch to iterate.

After some offline discussion, this feature will not change the semantics of the current operator reading storage, and mem table spill only uses the last 12 bits of epoch (spill offset <= 4096) can be acceptable after Global memory statistics and dynamic flush threshold for mem table is implemented. We only need to change the write logic when building SSTs, limiting the spill offset up to 4096 times when writing.

Let's start this change after global memory statistics and dynamic threshold is implemented.

[github-actions]

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