In rare cases DiskWriter::run() may call finish_capture()
concurrently with the butler thread from transport_stopped_wallclock,
this can lead to memory corruption (CaptureInfo).
Using a Mutex here is not great, but it is not usually contended
and better than crashing at rec-stop.
We should probably change DiskWrWiter::_was_recording into an
atomic bool, and use CAS to prevent concurrent calls.
fbfeff4168