Max purgeable has two possible values for each partition: one for
regular tombstones and one for shadowable ones. Yet currently a single
member is used to cache the max-purgeable value for the partition, so
whichever kind of tombstone is checked first, its max-purgeable will
become sticky and apply to the other kind of tombstones too. E.g. if the
first can_gc() check is for a regular tombstone, its max-purgeable will
apply to shadowable tombstones in the partition too, meaning they might
not be purged, even though they are purgeable, as the shadowable
max-purgeable is expected to be more lenient. The other way around is
worse, as it will result in regular tombstone being incorrectly purged,
permitted by the more lenient shadowable tombstone max-purgeable.
Fix this by caching the two possible values in two separate members.
A reproducer unit test is also added.
Fixes: scylladb/scylladb#23272Closesscylladb/scylladb#24171
7db956965e