Currently row marker shadowing the shadowable tombstone is only checked
in `apply(row_marker)`. This means that shadowing will only be checked
if the shadowable tombstone and row marker are set in the correct order.
This at the very least can cause flakyness in tests when a mutation
produced just the right way has a shadowable tombstone that can be
eliminated when the mutation is reconstructed in a different way,
leading to artificial differences when comparing those mutations.
This patch fixes this by checking shadowing in
`apply(shadowable_tombstone)` too, making the shadowing check symmetric.
There is still one vulnerability left: `row_marker& row_marker()`, which
allow overwriting the marker without triggering the corresponding
checks. We cannot remove this overload as it is used by compaction so we
just add a comment to it warning that `maybe_shadow()` has to be manually
invoked if it is used to mutate the marker (compaction takes care of
that). A caller which didn't do the manual check is
mutation_source_test: this patch updates it to use `apply(row_marker)`
instead.
Fixes: #9483
Tests: unit(dev)
Closes#9519
6a76e12768