- Use `sstables::generation_type` in more places
- Enforce conceptual separation of `sstables::generation_type` and `int64_t`
- Fix `extremum_tracker` so that `sstables::generation_type` can be non-default-constructible
Fixes#10796.
Closes#10844
* github.com:scylladb/scylla:
sstables: make generation_type an actual separate type
sstables: use generation_type more soundly
extremum_tracker: do not require default-constructible value types