Add a ReaderLock to Route::process_output_buffers().
But process_output_buffers() is always called with processor-lock
held. To avoid deadlocks, a processor WriterLock must always imply
a process-lock (IFF reconfigure-I/O is called with _processor_lock).
Otherwise: e.g.
* add_processor() -> takes processor-lock. set up and activate processor.
* simult. audio-engine process, process-lock -> call process_output_buffers() -> wait for processor-lock
* add_processor() continues -> calls reconfigure-io -> take process-lock -> deadlock.
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