Enable `RocksDB` backend for `AccountTreeWithHistory`

[drahnr]

[bobbinth]

As a part of this, we should also consider getting rid of the AccountTreeStorage trait. We should be able to do it by defining, the struct as something like this:

pub struct AccountTreeWithHistory<S: SmtStorage> {
    block_number: BlockNumber,
    latest: AccountTree<S>,
    overlays: BTreeMap<BlockNumber, HistoricalOverlay>,
}

[bobbinth]

@drahnr - is this addressed already, or is there anything outstanding?

[drahnr]

Done, nothing outstanding to my knowledge