Critical Description
If an ordered scan starts during the 'phase 2' of a compaction (when there is a DuplicatingRowSet in place), then the DuplicatingRowSet ends up creating a UnionIterator of the original rowsets. This union is non-ordered, so it ends up yielding out-of-order rows.
This is causing alter_table-randomized-test to have verification failures about 0.25% of the time, but should be relatively easy to reproduce by adding concurrent ordered scans to some stress test that causes lots of compaction.