NBO applying

- High priority interface method to apply NBO begin, separate from
  apply_toi() in order to avoid implementation to force interpreting
  ws_meta flags.
- Method to put client_state into NBO mode when applying NBO begin.
  The client_state will process in m_local mode.
- Unit tests for applying NBO

include/wsrep/client_state.hpp

@@ -755,6 +755,18 @@ namespace wsrep
          */
         int end_nbo_phase_one();
 
+        /**
+         * Enter in NBO mode. This method should be called when the
+         * applier launches the asynchronous process to perform the
+         * operation. The purpose of the call is to adjust
+         * the state and set write set meta data.
+         *
+         * @param ws_meta Write set meta data.
+         *
+         * @return Zero in case of success, non-zero on failure.
+         */
+        int enter_nbo_mode(const wsrep::ws_meta& ws_meta);
+
         /**
          * Begin non-blocking operation phase two. The keys argument
          * passed to this call must contain the same keys which were