flow is now visible if either both peer topics are non-virtual or if
the local topic is virtual: this means that virtual topics see other
virtual topics come and go as well as normal topics, but normal topics
only see other normal topics. Adjusting the example program's session
manager to subscribe virtually and then only pay attention to new
normal flows solves the socket-shutdown problem: because all the
wildcard, resource-managing peers are now virtually subscribed and the
resources themselves are normally subscribed, the resources can use
normal presence to manage their own lifecycles.
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