Core/Instance: stop updating the instance resettimes based on creature respawns

- Rather update normal instance reset time to 2 hours after last creature kill
- This fixes yet another integer overflow due to the possibility of having time_t max showing up
- Also change respawntime and resettime fields to bigint on respawn/instance related tables
- Start using prepared statements on the InstanceSaveMgr

src/server/game/Entities/Unit/Unit.cpp

@@ -12132,7 +12132,7 @@ void Unit::Kill(Unit* victim, bool durabilityLoss)
                 {
                     // the reset time is set but not added to the scheduler
                     // until the players leave the instance
-                    time_t resettime = creature->GetRespawnTimeEx() + 2 * HOUR;
+                    time_t resettime = GameTime::GetGameTime() + 2 * HOUR;
                     if (InstanceSave* save = sInstanceSaveMgr->GetInstanceSave(creature->GetInstanceId()))
                         if (save->GetResetTime() < resettime)
                             save->SetResetTime(resettime);