Core/Combat: reset the ThreatManager update interval upon engaging the creature and move ThreatClear packet sending into the update cycle to mimic retail behavior

Port From (https://github.com/TrinityCore/TrinityCore/commit/24bda9c73dcd89d4da4e871e00fd83a24b8275d2)

Source/Game/Entities/Creature/Creature.cs

@@ -466,6 +466,8 @@ namespace Game.Entities
 
             UpdateMovementCapabilities();
 
+            GetThreatManager().Update(diff);
+
             switch (m_deathState)
             {
                 case DeathState.JustRespawned:
@@ -541,8 +543,6 @@ namespace Game.Entities
                     if (!IsAlive())
                         break;
 
-                    GetThreatManager().Update(diff);
-
                     if (_spellFocusInfo.Delay != 0)
                     {
                         if (_spellFocusInfo.Delay <= diff)
@@ -1071,6 +1071,8 @@ namespace Game.Entities
         {
             base.AtEngage(target);
 
+            GetThreatManager().ResetUpdateTimer();
+
             if (!HasFlag(CreatureStaticFlags2.AllowMountedCombat))
                 Dismount();