delay freeze of status_cache until squash (#3453)

runtime/src/bank.rs

@@ -261,7 +261,11 @@ impl Bank {
 
         let parent_caches: Vec<_> = parents
             .iter()
-            .map(|b| b.status_cache.read().unwrap())
+            .map(|p| {
+                let mut parent = p.status_cache.write().unwrap();
+                parent.freeze();
+                parent
+            })
             .collect();
         self.status_cache.write().unwrap().squash(&parent_caches);
     }