Cope with possibility of duplicate uuid, in e.g. queue/fanout/direct.
If a subscription token matches an existing subscription, there's a
collision, so choose a new token until there's no collision.

Factor out commonality in subscription-management from queue/fanout/direct.

SAX-style sexp reader/writer, so that we can do something sensible for
enormous (e.g. gigabyte-sized) messages.