Core/Spells: Fix spelleffect sanctuary

- No longer removes every attacker, but updates the visibility
  - Using that effect won't remove attackers that can still see through the stealth of the caster based on distance
- Ignores spells that were casted before or at the same time of that effect
- Trigger other needed spells when casting Shadowmeld

--HG--
branch : trunk

src/server/game/Entities/Unit/Unit.h

@@ -1774,6 +1774,7 @@ class Unit : public WorldObject
         void UpdateObjectVisibility(bool forced = true);
 
         SpellImmuneList m_spellImmune[MAX_SPELL_IMMUNITY];
+        uint32 m_lastSanctuaryTime;
 
         // Threat related methods
         bool CanHaveThreatList() const;