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

dbsim/db_high_priority_service.cpp

@@ -63,6 +63,13 @@ int db::high_priority_service::apply_toi(
     throw wsrep::not_implemented_error();
 }
 
+int db::high_priority_service::apply_nbo_begin(
+    const wsrep::ws_meta&,
+    const wsrep::const_buffer&)
+{
+    throw wsrep::not_implemented_error();
+}
+
 int db::high_priority_service::commit(const wsrep::ws_handle& ws_handle,
                                       const wsrep::ws_meta& ws_meta)
 {