In order for this method to make sense as an async, the returned box
must have its lifetime tied to the store. This did not jive well with
the mock implementation of origin::Store, so I ditched that, along with
the mux tests which depended on it. Oh well.
This reverts commit a86020eedf.
Turns out that even with this change proxying won't work properly via
origin::Store, it'll just have to be a special case. I'm keeping the
commit around in case we want this for a later case.
