Implement a transaction mechanism

In order to support buying and selling things, as well as trading with other players, there needs to be a "transaction" mechanism.
A Transact operation starts the transaction, between two entities. While it's active, both can enter into the "transaction" instance the items that they want to transfer to the other party. Once one of the entities have "closed" the transaction it can't be altered. The other entity now either have to close or abort the transaction. When this happens server side code makes sure to move the pledged entities between the entities.