Leveled list merchant containers respawn on barter

To give an example: if you go to Galdebir in Balmora Mages Guild, open her chest and loot it, then barter with her, new random loot will appear. This behavior can be abused and is not present in the original game.

(RM-3929 from redmine: created on 2017-06-24 by Anton Uramer, , relates #3862 (closed))