When bouncing Region or Range, Session::write_one_track()
blocks processing, but takes no process-lock.
It is possible that a latency-callback arrives at the same
time while Route::bounce_process is active and calls ::run.
This can trigger a delayline.cc Assertion `lm.locked ()' failed
in either thread.
Now latency-callbacks are postponed until the session can
process normally again
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