Load block roots from fork choice where possible when serving BlocksByRange requests (#7058)

* Load block roots from fork choice where possible to avoid loading state from disk when serving block by range requests.

* Check if the start slot is newer than finalization (`start_slot >= finalized_slot`), and use fork choice in that case.

consensus/proto_array/src/proto_array_fork_choice.rs

@@ -856,10 +856,18 @@ impl ProtoArrayForkChoice {
     }
 
     /// See `ProtoArray::iter_nodes`
-    pub fn iter_nodes<'a>(&'a self, block_root: &Hash256) -> Iter<'a> {
+    pub fn iter_nodes(&self, block_root: &Hash256) -> Iter {
         self.proto_array.iter_nodes(block_root)
     }
 
+    /// See `ProtoArray::iter_block_roots`
+    pub fn iter_block_roots(
+        &self,
+        block_root: &Hash256,
+    ) -> impl Iterator<Item = (Hash256, Slot)> + use<'_> {
+        self.proto_array.iter_block_roots(block_root)
+    }
+
     pub fn as_bytes(&self) -> Vec<u8> {
         SszContainer::from(self).as_ssz_bytes()
     }