SfxMedium already had a m_bDisableFileSync member; if the medium has a
storage, then forward this flag to it, so at the end
SwitchablePersistenceStream::waitForCompletion() (and the called
fileaccess::XStream_impl::waitForCompletion()) does not call
osl_syncFile(), either.

Times for 100 hello world inputs: 12594 -> 5281 ms is spent in XHTML-load + ODT
export + close (42% of original).

Change-Id: I2aab6c9e6baf133b211620004dcea66bd41ffc6f
Reviewed-on: https://gerrit.libreoffice.org/47766Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>