component AP gateway: AP delete activity => message retract: handle retractation of messages. As it is not possible to know from the AP item alone if we need to to a message retractation (XEP-0424) or a pubsub retractation (XEP-0060), we now cache sent message, and decide which method to use according to how the item is cached (i.e. in message history or in pubsub cache). rel 367