This PR contains some small improvements to the safety of consuming/releasing resources to/from the semaphore:
* reader_permit: make the low-level `consume()/signal()` API private, making the only user (an RAII class) friend.
* reader_resources: split `reset()` into `noexcept` and potentially throwing variant.
* reader_resources::reset_to(): try harder to avoid calling `consume()` (when the new resource amount is smaller then the previous one)
Closes#13678
* github.com:scylladb/scylladb:
reader_permit: resource_units::reset_to(): try harder to avoid calling consume()
reader_permit: split resource_units::reset()
reader_permit: make consume()/signal() API private
97694d26c4