This fixes a regression introduced by 1e7a444, where table::get_sstable_set() isn't exposing all sstables, but rather only the ones in the main set. That causes user of the interface, such as get_sstables_by_partition_key() (used by API to return sstable name list which contains a particular key), to miss files in the maintenance set.
Fixes https://github.com/scylladb/scylladb/issues/11681.
Closes#11682
* github.com:scylladb/scylladb:
replica: Return all sstables in table::get_sstable_set()
sstables: Fix cloning of compound_sstable_set