core/state: use dirty flag to avoid db writes

Until now the dirty flag was unset in intermediate root. That resulted
in not knowing whether a state object should be writte to disk or not.
Being removed in 1.4.12, we can rely on it to avoid a ton of writes.

core/state/statedb.go

@@ -445,7 +445,7 @@ func (s *StateDB) commit(dbw trie.DatabaseWriter) (root common.Hash, err error)
 			// and just mark it for deletion in the trie.
 			s.DeleteStateObject(stateObject)
 			delete(s.all, addr)
-		} else {
+		} else if stateObject.dirty {
 			// Write any contract code associated with the state object
 			if stateObject.code != nil {
 				if err := dbw.Put(stateObject.CodeHash(), stateObject.code); err != nil {