Summary:
Pull Request resolved: https://github.com/facebook/rocksdb/pull/14876
Add reverse scan support to `MultiScanArgs`/`MultiScan` so callers can iterate bounded scan ranges in descending order. Plumb reverse scan state through `DBIter`, `BlockBasedTableIterator`, and `MultiScanIndexIterator`, including reverse prefetch windowing and async I/O support. Update `db_bench` `multiscanrandom` to honor `--reverse_iterator` and report rows scanned so reverse MultiScan and iterator baselines can be compared on row throughput.
Add `db_stress --multiscan_reverse` coverage so the prepared MultiScan path can be stressed with `SeekForPrev()`/`Prev()` against a control iterator.
Also fix the table child iterator path to accept reverse `SeekForPrev()` targets above a file's prepared ranges. `MergingIterator` and `LevelIterator` can legally seek every child to the current range limit; a child file may only have lower prepared ranges, so treating that as invalid caused intermittent `SeekForPrev target is outside prepared ranges` exceptions in the benchmark.
Reviewed By: xingbowang
Differential Revision: D109357730
fbshipit-source-id: eccece5ca7d8f5d6760d51ebe76801a332a49061
e1ea04d9ef