flush pending Session Events at session close

flush pending Session Events at session close

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.