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/high_priority_service.hpp

@@ -161,6 +161,25 @@ namespace wsrep
                               const wsrep::const_buffer& ws,
                               wsrep::mutable_buffer& err) = 0;
 
+        /**
+         * Apply NBO begin event.
+         *
+         * The responsibility of the implementation is to start
+         * an asynchronous process which will complete the operation.
+         * The call is done under total order isolation, and the
+         * isolation is released by the caller after the method
+         * returns. It is a responsibility of the asynchronous process
+         * to complete the second phase of NBO.
+         *
+         * @param ws_meta Write set meta data.
+         * @param data Buffer containing the command to execute.
+         *
+         * @return Zero in case of success, non-zero if the asynchronous
+         *         process could not be started.
+         */
+        virtual int apply_nbo_begin(const wsrep::ws_meta& ws_meta,
+                                    const wsrep::const_buffer& data) = 0;
+
         /**
          * Actions to take after applying a write set was completed.
          */