Currently when state pruning is enabled state-db reqeuires O(n) memory w.r.t. the number of blocks pending pruning. Therefore long pruning history requires a lot of memory. It should be possible to get rid of the memory requirement using the reference counting feature of parity-db.

This basically means removing death_rows and death_index fields in RefWindow struct. Journals that are currently kept in death_rows should be loaded on demand from the database, although I'd keep an in-memory cache for pruning window size <= 256. death_index should be replace with reference counting on the DB level.