Introduce states pruning

[kamilsa]

Qlean stores all states in memory in ForkChoice's

std::unordered_map<BlockHash, State> states_;

This leads to OOM issue: leanEthereum/pm#50

As a workaround we should replace unordered_map with StatesStorage that will store all states and be able to prune latest states. StateStorage is an interface, which for now will only have in-memory implementation. The number of states that we should store in StatesStorage should be configurable via --states-pruning which should accept either the number of latest states or archive which would disable pruning