Review fixes

* Move resetting is_bf_immutable_ into trasaction::cleanup()
  to ensure that it is reset to false regardless how the
  transaction terminates.
* Removed redundant lock()/unlock() methods from
  mock_client_state.

test/mock_client_state.hpp

@@ -49,8 +49,6 @@ namespace wsrep
                 (void)client_service().bf_rollback();
             }
         }
-      void lock() { mutex_.lock(); }
-      void unlock() { mutex_.unlock(); }
     private:
         wsrep::default_mutex mutex_;
         wsrep::default_condition_variable cond_;