This allows a TmpFile pointer to be either a Sync or Async (Threaded)
writer. As result we must be able to handle both RT and non RT processing.
Still, post-processing (normalization and encoding) should always
happen faster than realtime (freewheeling).
Since jack does not allow a client to change to freewheeling from within
the process-callback, the async-writer disk-thread FileFlushed is used
to initiate post-processing.
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