Wait for the stale object to become fully fetched, so that we can catch
fetch errors, before we unbusy the new object. This serves two
purposes. First it helps with request coalescing, and stops long chains of
IMS-updated short-TTL objects all streaming from a single slow body
fetch. Second it makes sure that all the object attributes are complete
when we copy them (this would be an issue for ie OA_GZIPBITS).

This patch OBE's r01646.vtc, and slightly patches r01648.vtc to expect a
503 instead of a 200 and a broken connection on the failing client.

Fixes: #3089