Avoid cycles in bean dumps

[sbordet]

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Jetty version(s) 12.1.x

Description EventListener added to a component may be copied to a subcomponent.

This is the case where we add e.g. a Connection.Listener to Connector, but then we copy the Connection.Listener to the actual Connection object every time the Connector creates a new one.

It may happen that one of the copied listeners is a container for the subcomponent, which will form a cycle (not in the bean graph, perhaps, but in the dump execution). For example, when HTTP2SessionContainer is added as a listener to HTTP2Session: trying to dump the session results in trying to dump the container, which tries to dump the session, resulting in StackOverflowError.

We should have a generic mechanism to avoid this dump cycle.