Route::realtime_handle_transport_stopped() does have insufficient
information (PostTransportLocate), so "flush" is called from
Route::non_realtime_transport_stop in the butler thread.
However plugin de/activate() must not be called concurrently with
processing. e.g. https://lv2plug.in/ns/lv2core explicitly states:
"Hosts MUST guarantee that: An Instantiation function for an instance
is never called concurrently with any other function for that instance."
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