Loot Handler fix, last revision

- Fixing bugs on loot handler for groups looting modes.
- Send now message to the client whenever he must not be able to pick-up the loot.
- When assigned to a player, an item in master loot mode can't be stolen anymore.
- When a roll is won by a player for a given object, nobody else can get the loot anymore.
- Fixing a core crash bug with Roll for Rare Vein mechanism.