Allow private messages to be deleted
Feedback from a user: "We should be able to delete our half of private messages. When the second party deletes their half of the conversation it should be completely deleted."
Private messages include invitations and responses in sharing and introduction protocols. Forum, blog and private group clients all depend on the invitation message being available when the accept message is received, so they can get the group descriptor from the invitation message in order to add the group.
We can easily migrate that information into the session. For this we need a way for clients to run migrations. The first phase of the message deletion work would be to add some kind of client migration mechanism.