For chunked encoding, we do not know how big the object is ultimately
going to be, so VFP_GetStorage() called ObjGetSpace() with the
fetch_chunksize parameter in this case.

Yet which size is best might differ for different storage engines, and
having the information that the caller does not know the final size
might be relevant. Storage engines could guess that if a request came
in for fetch_chunksize that this _might_ be the "chunked" case, but
that heuristic would be wrong for Objects of just that size advertised
via Content-Length.

So this patch takes the guesswork out of the game by just passing the
magic 0 value down to the storage engine to mean "give me some good
chunk of bytes, I do not know how much I am going to need".