feat(Core/Grids): Implement visibility notifier (#15919)

* Cherry-picked from TrinityCore (unable to find author)

src/server/game/Grids/NGrid.cpp

@@ -0,0 +1,29 @@
+/*
+ * This file is part of the AzerothCore Project. See AUTHORS file for Copyright information
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU Affero General Public License as published by the
+ * Free Software Foundation; either version 3 of the License, or (at your
+ * option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "NGrid.h"
+#include "Random.h"
+
+GridInfo::GridInfo() : i_timer(0), vis_Update(0, irand(0, DEFAULT_VISIBILITY_NOTIFY_PERIOD)),
+i_unloadActiveLockCount(0), i_unloadExplicitLock(false), i_unloadReferenceLock(false)
+{
+}
+
+GridInfo::GridInfo(std::chrono::seconds expiry, bool unload /*= true */) : i_timer(expiry.count()), vis_Update(0, irand(0, DEFAULT_VISIBILITY_NOTIFY_PERIOD)),
+i_unloadActiveLockCount(0), i_unloadExplicitLock(!unload), i_unloadReferenceLock(false)
+{
+}