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/Handlers/CharacterHandler.cs

@@ -608,7 +608,7 @@ namespace Game
                     stmt = LoginDatabase.GetPreparedStatement(LoginStatements.REP_REALM_CHARACTERS);
                     stmt.AddValue(0, createInfo.CharCount);
                     stmt.AddValue(1, GetAccountId());
-                    stmt.AddValue(2, Global.WorldMgr.GetRealm().Id.Index);
+                    stmt.AddValue(2, Global.RealmMgr.GetCurrentRealmId().Index);
                     loginTransaction.Append(stmt);
 
                     DB.Login.CommitTransaction(loginTransaction);