The use-case is when the consumer of the output file will read it right
after SfxBaseModel::storeToURL() returns, in which case an expensive
fsync is pointless.

Times for 100 hello world inputs: 8516 -> 2785 ms is spent in ODT-load + HTML
export + close (33% of original).

Change-Id: I05e424a43ebfeea363f82b57af60f5aaa28696b4
Reviewed-on: https://gerrit.libreoffice.org/47695Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>