This prevents a memory leak: The session is closed.
The session-butler drops memory pool trash. The Engine keeps running.
Once the AudioEngine is taken down (sample-rate switch, or at exit),
the backend process-thread is terminated but there is no trash-can anymore.
If there are unprocessed SessionEvents, this calls
free_per_thread_pool() -> cp->parent()->add_to_trash (cp).
"parent()" in this case the trash-can the butler emptied.
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