Implement forwards iterator for state roots

[paulhauner]

Description

As per #2376, it turns out that deep reverse iterators are causing big memory spikes.

Whilst #2376 addresses this issue for block root lookups, it'll still exist for state roots. Whilst I don't think this is really a big deal for "normal operation" (i.e., following the chain and running validators), it'll mean the calls via the HTTP API to older state roots will be slow and expensive for memory.

One way to solve this would be to just implement an O(1) state root lookup using the freezer chunked roots. However, since we already have the foward iters for block roots then perhaps it's easier to adapt them to also do state roots. That way we also get iteration, if we want. I have no strong preference.

[paulhauner]

I've given @macladson a quick brief on this one. It's might be a bit difficult, but I think it's worth at least having a look this Mac 🙂 Don't hesitate to holler if it's too painful.

[michaelsproul]

Closed by #2422