`ethexe`: implement a cleanup database util

[grishasobol]

Problem to Solve

Database size on Vara.eth testnet is already bigger than 3 GB. We should start to think about cleanup util. Current approach of database usage involves many intermediate states saving to CAS, that states could be removed

Possible Solution

1. Validator stopped
2. Validator started with flag --clean-up. That cause a clean-up util to start before main validator service started.
3. Util chooses latest (or almost latest, it's not so important, but newer == better) finalised and computed announce. All program states are known for this announce. So, we should iterate and collect all hashes in use and delete all other hashes which are not in use already.
4. Should take into account that this announce must be marked as start now, because after that validator cannot operate safely with any data before this announce and corresponding block

Notes

No response