Core/Spell Mechanics: introduce global cooldown manager for server-side checks for GCD (ported from mangos)

src/server/game/Spells/Spell.h

@@ -543,6 +543,9 @@ class Spell
 
         void SetSpellValue(SpellValueMod mod, int32 value);
     protected:
+        bool HasGlobalCooldown();
+        void TriggerGlobalCooldown();
+        void CancelGlobalCooldown();
 
         void SendLoot(uint64 guid, LootType loottype);