Port and connection changes always imply a latency_callback from
the engine. Worst I/O latency is updated directly in
Session::update_latency() which is called from
AudioEngine::latency_callback.
Explicit subscriptions to route->output()->changed() is not needed
to update the worst I/O latency.
Only set_block_size() needs to to update the I/O latency when
the buffer-size changes.
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