[sheepdog] async flush in the object cache

[Christoph Hellwig]

I startes to look a bit over the object cache, and the async cache
flush option scares me.  It only replies to the request before committing
data to the backing storage, thus breaking the transactional sematics
of any filesysem using the volume, as well as removing the possibily
to return errors for the cache flush.

What is the intended use case for it?  I would be much more happy if
this breakage of semantics wasn't offered at all.