Retain only candidates that should be purged or with non-empty slot l…

…ist (#2679)

* Retain only candidates that should be purged or with non-empty slot list

* Clean accounts older than root while scanning candidates for purging

* Clippy fix

* Rename clean_accounts_older_than_root

* Add retained_keys_count metric

* Split clean_account_older_than_root into collecting and handling parts

* Implement feedback

* Remove debug

* Add debug_assert

* Check reclaims