Allow create_pending_deletion_log to delete a bunch of sstables
potentially resides in different prefixes (e.g. in the base directory
and under staging/).
The motivation arises from table::cleanup_tablet that calls compaction_group::cleanup on all cg:s via cleanup_compaction_groups. Cleanup, in turn, calls delete_sstables_atomically on all sstables in the compaction_group, in all states, including the normal state as well as staging - hence the requirement to support deleting sstables in different sub-directories.
Also, apparently truncate calls delete_atomically for all sstables too, via table::discard_sstables, so if it happened to be executed during view update generation, i.e. when there are sstables in staging, it should hit the assertion failure reported in https://github.com/scylladb/scylladb/issues/18862 as well (although I haven't seen it yet, but I see no reason why it would happen). So the issue was apparently present since the initial implementation of the pending_delete_log. It's just that with tablet migration it is more likely to be hit.
Fixesscylladb/scylladb#18862
Needs backport to 6.0 since tablets require this capability
Closesscylladb/scylladb#19555
* github.com:scylladb/scylladb:
sstable_directory: create_pending_deletion_log: place pending_delete log under the base directory
sstables: storage: keep base directory in base class
sstables: storage: define opened_directory in header file
sstable_directory: use only dirlog
a4a8cad97f