This PR changes the following to speed up L0 compaction and
prevent OOM:
1. Lower deltabuf limit to 16MB by default, so that each L0 segment
would be 4X smaller than before.
2. Add BatchProcess, use it if memory is sufficient
3. Iterator will Deserialize when called HasNext to avoid massive memory
peek
4. Add tracing in spiltDelta
See also: #30191
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Signed-off-by: yangxuan <xuan.yang@zilliz.com>