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

src/client_state.cpp

@@ -500,6 +500,17 @@ int wsrep::client_state::end_nbo_phase_one()
     return ret;
 }
 
+int wsrep::client_state::enter_nbo_mode(const wsrep::ws_meta& ws_meta)
+{
+    assert(state_ == s_exec);
+    assert(mode_ == m_local);
+    wsrep::unique_lock<wsrep::mutex> lock(mutex_);
+    toi_meta_ = ws_meta;
+    toi_mode_ = mode_;
+    mode(lock, m_nbo);
+    return 0;
+}
+
 int wsrep::client_state::begin_nbo_phase_two(const wsrep::key_array& keys)
 {
     assert(state_ == s_exec);

src/server_state.cpp

@@ -463,8 +463,10 @@ static int apply_toi(wsrep::provider& provider,
     }
     else if (wsrep::starts_transaction(ws_meta.flags()))
     {
-        // NBO begin
-        throw wsrep::not_implemented_error();
+        provider.commit_order_enter(ws_handle, ws_meta);
+        int ret(high_priority_service.apply_nbo_begin(ws_meta, data));
+        provider.commit_order_leave(ws_handle, ws_meta);
+        return ret;
     }
     else if (wsrep::commits_transaction(ws_meta.flags()))
     {