Store latest timestamp and unread count in group metadata for invitations
The contact list loads all invitation messages just to get the latest timestamp and unread message count for each conversation. This is a very expensive provess that involves loading the session state for every message, which in the worst case iterates over all sessions. Store this information in the group metadata and expose it through the SharingManager interface.
The timestamp should relate to the latest message that would be visible in the UI, i.e. invitations and responses.
Related to #373 (closed).