Optimization: avoid recomputing known state roots (#762)

* Start adding optimization

* Add temp fix for protobuf issue

* Fix compile errors

* Fix protobuf import

beacon_node/beacon_chain/src/test_utils.rs

@@ -287,7 +287,7 @@ where
         }
 
         while state.slot < slot {
-            per_slot_processing(&mut state, &self.spec)
+            per_slot_processing(&mut state, None, &self.spec)
                 .expect("should be able to advance state to slot");
         }