Core/Realms: Realmlist refactors

* Removed global realm variable from World and use RealmList everywhere
* Match auth build key with client version
* Restored allowedSecurityLevel checks for realmlist packet building
* Restored updating population field
Port From (https://github.com/TrinityCore/TrinityCore/commit/c4b710446d62c95eb8124175203fa5f394912594)

Source/Game/Entities/Object/ObjectGuid.cs

@@ -576,7 +576,7 @@ namespace Game.Entities
             if (realmId != 0)
                 return realmId;
 
-            return Global.WorldMgr.GetRealmId().Index;
+            return Global.RealmMgr.GetCurrentRealmId().Index;
         }
     }