Otherwise regular compaction can sneak in and
see !cs.sstables_requiring_cleanup.empty() with
cs.owned_ranges_ptr == nullptr and trigger
the internal error in `compaction_task_executor::compact_sstables`.
Fixes scylladb/scylladb#14296
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Closes#14297