Otherwise, we waste a request on some 416/206 response that won't be
helpful.
To do this, add a new X-Backend-Ignore-Range-If-Metadata-Present header
whose value is a comma-separated list of header names. Middlewares may
include this header to tell object-servers to send the whole object
(rather than a 206 or 416) if *any* of the metadata are present.
Have dlo and symlink use it, too; it won't save us any round-trips, but
it should clean up some object-server logging.
Change-Id: I4ff2a178d0456e7e37d561109ef57dd0d92cbd4e
e8b654f318