Under load, client c4 from g00005.vtc may fail with a 200 response
instead of the expected 206 partial response.

There is a window during which we might still see a boc, but because
c4 sets beresp.do_stream to false, the fetch has to be over. To close
this race we can instead reference the boc as suggested in #2904 and
keep track of the boc state.