Fix buglet
Fix a fine point in this test-case:  We can get a Content-Length on
a 304 response, but we still don't get the body.
Fix other 304 return cases to not expect a body
IMS decision is protocol-independent
Single test for Range-qualifying request
Move the stream-stopper all the way to where streaming will happen
Atomically grab the busyobj, if there is one.
Stream.

(Don't tell me nothing good ever comes from DB's delayed trains...)