The connection close listener initiates a presence broadcast to
advertise its "unavailable" status. The current connection must be
marked closed before this occurs, so the presence stanza will not be
delivered to the newly closed connection.

Note that the synchronized block alone does not accomplish this because
the presence broadcast is delivered via that same thread that is
currently closing the connection, thus it already possesses the sync
object lock.

Also, note that the SessionManager should exclude the originating JID
when sending presence broadcasts to the user's other available
session(s).