This happened initially during session load.
The GUI thread performed a direct refill (blocking wait)
`Session::post_engine_init() -> Track::seek() -> DiskReader::seek() -> DiskReader::do_refill_with_alloc()`
while concurrently the butler thread does the same:
```
Session::butler_transport_work() -> Track::non_realtime_locate() -> Route::non_realtime_locate()
-> DiskIOProcessor::non_realtime_locate() -> DiskReader::seek() -> DiskReader::do_refill_with_alloc()
-> DiskReader::refill_audio()
```
We do not want the GUI to wait, so now we just request a locate
and let refill happen in the background.
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