Flush() turns into a sync() on the device, which is pretty slow.
Most of the time all we actually want to do is to flush the internal
buffers from the SvStream.
So expose the FlushBuffer method and use that where possible.
And also means we don't need the mbDontFlushOnClose flag on SvStream
any more.
Change-Id: Ibe089b88b325f0fdc565200a97ce72cd26b7fcfb
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/128214
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
17950f4d20