It was possible that auto-connect-thread or GUI thread called
DelayLine::set_delay without acquiring the process-lock.
Changing the delay while processing is not supported and
can cause glitches.
backtrace:
```
DelayLine::set_delay
Send::update_delaylines
Send::set_delay_out
InternalReturn::set_playback_offset
Route::update_signal_latency
Session::update_route_latency
Session::update_latency_compensation
Session::auto_connect_thread_run
```
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