Better messaging between WFAs
Add some kind of metadata-messaging support. For instance, two WFAs have no way to coordinate mutual activities other than to hand caps back and forth. We both need to add support for arbitrary (or sanitized arbitrary) metadata attached to each cap sent through an RP or MB, and we need to be able to send messages not attached to caps. Presumably the latter would take the form of a mailbox/dropbox where each side of the mailbox can leave messages for the other side. That is sort of ugly, but it may also be good to have a mediated communication abstraction.