Makes sstable_set::all() interface robust, and introduces sstable_set::size() to avoid copies when retrieving set size.
Closes#12716
* github.com:scylladb/scylladb:
treewide: Use new sstable_set::size() wherever possible
sstables: Introduce sstable_set::size()
sstables: Fix fragility of sstable_set::all() interface