handling 1xx responses

Heres my current thoughts:

We rework the data path to separate metadata from data as per the
current architecture TODO.

Then, we change the rule that says a request can only have one object
returned - we can send a metadata with no body and a flag (say
'interim') down the pipeline. Whoever originated the cause of the 1xx
reply swallows it, and the store effectively ignores it because of the
interim flag.

Thoughts?

Rob