Iterators are long deprecated, but sort are still using it. This PR
unifies stats task with the latest compaction common functions and
remove the usage of iterators.
1. Rename `datanode/compaction` to `datanode/compactor`
2. Add `internal/compaction` and move some compaction commons into it.
3. Replace `DeltalogIterators` with `ComposeDeleteFromDeltalogs`
4. Remove `datanode/iterators`
See also: #39242
Signed-off-by: yangxuan <xuan.yang@zilliz.com>
XuanYang-cn