See also #40558
Related to #35303 & #38066 as well
This PR:
- Add `BufferedForward` to limit memory usage forwarding stream delete
- Add `UseLoad` flag to determine `Delete` shall use `segment.Delete` or
`segment.LoadDelta`
- Fix delegator accidentally use always true candidate while load
streaming delta
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Signed-off-by: Congqi Xia <congqi.xia@zilliz.com>
