Use forwards iterator for state root lookups (#2422)

## Issue Addressed

#2377 

## Proposed Changes

Implement the same code used for block root lookups (from #2376) to state root lookups in order to improve performance and reduce associated memory spikes (e.g. from certain HTTP API requests).

## Additional Changes

- Tests using `rev_iter_state_roots` and `rev_iter_block_roots` have been refactored to use their `forwards` versions instead.
- The `rev_iter_state_roots` and `rev_iter_block_roots` functions are now unused and have been removed.
- The `state_at_slot` function has been changed to use the `forwards` iterator.

## Additional Info

- Some tests still need to be refactored to use their `forwards_iter` versions. These tests start their iteration from a specific beacon state and thus use the `rev_iter_state_roots_from` and `rev_iter_block_roots_from` functions. If they can be refactored, those functions can also be removed.

beacon_node/store/src/hot_cold_store.rs

@@ -2,7 +2,7 @@ use crate::chunked_vector::{
     store_updated_vector, BlockRoots, HistoricalRoots, RandaoMixes, StateRoots,
 };
 use crate::config::{OnDiskStoreConfig, StoreConfig};
-use crate::forwards_iter::HybridForwardsBlockRootsIterator;
+use crate::forwards_iter::{HybridForwardsBlockRootsIterator, HybridForwardsStateRootsIterator};
 use crate::impls::beacon_state::{get_full_state, store_full_state};
 use crate::iter::{ParentRootBlockIterator, StateRootsIterator};
 use crate::leveldb_store::BytesKey;
@@ -393,6 +393,16 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
         HybridForwardsBlockRootsIterator::new(store, start_slot, end_state, end_block_root, spec)
     }
 
+    pub fn forwards_state_roots_iterator(
+        store: Arc<Self>,
+        start_slot: Slot,
+        end_state_root: Hash256,
+        end_state: BeaconState<E>,
+        spec: &ChainSpec,
+    ) -> Result<impl Iterator<Item = Result<(Hash256, Slot), Error>>, Error> {
+        HybridForwardsStateRootsIterator::new(store, start_slot, end_state, end_state_root, spec)
+    }
+
     /// Load an epoch boundary state by using the hot state summary look-up.
     ///
     /// Will fall back to the cold DB if a hot state summary is not found.