DiskWriter::seek() flushes all recorded audio/MIDI, and resets
buffers as if no recording has happened.
Data must be flushed to disk before any non-realtime locates happen.
This happens at transport-stop, which is usually called first.
Except since 8b048bb351 Session::butler_transport_work() may call
non_realtime_stop() after a non_realtime_locate() when looping
more than once.
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