Core/Combat: Fix Evade Mechanics

- Can now kite mobs beyond ThreatRadius (CONF) if continuing to damage them. World bosses still use ThreatRadius to prevent pulling them to cities
- A mob that is beyond the ThreatRadius, it will return home (evade) after 10 seconds of no damage if there are no valid targets within attack distance of it, otherwise it will still attack
- Mobs now properly report "Evade" if attacked while returning home.

(modified - Nayd)

Closes #830
Closes #7748

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

@@ -173,6 +173,7 @@ Unit::Unit(bool isWorldObject): WorldObject(isWorldObject)
     , m_vehicleKit(NULL)
     , m_unitTypeMask(UNIT_MASK_NONE)
     , m_HostileRefManager(this)
+    , _lastDamagedTime(0)
 {
 #ifdef _MSC_VER
 #pragma warning(default:4355)
@@ -12416,6 +12417,10 @@ int32 Unit::ModifyHealth(int32 dVal)
     if (dVal == 0)
         return 0;
 
+    // Part of Evade mechanics. Only track health lost, not gained.
+    if (dVal < 0 && GetTypeId() != TYPEID_PLAYER && !isPet())
+        SetLastDamagedTime(time(NULL));
+
     int32 curHealth = (int32)GetHealth();
 
     int32 val = dVal + curHealth;
@@ -13042,6 +13047,14 @@ Unit* Creature::SelectVictim()
         return target;
     }
 
+    // Case where mob is being kited.
+    // Mob may not be in range to attack or may have dropped target. In any case,
+    //  don't evade if damage received within the last 10 seconds
+    // Does not apply to world bosses to prevent kiting to cities
+    if (!isWorldBoss() && !GetInstanceId())
+        if (time(NULL) - GetLastDamagedTime() <= MAX_AGGRO_RESET_TIME)
+            return target;
+
     // last case when creature must not go to evade mode:
     // it in combat but attacker not make any damage and not enter to aggro radius to have record in threat list
     // for example at owner command to pet attack some far away creature