Check object segment MD5s

Now if a client is downloading a large object and one of the segment
fetches gets bad data, we'll abort the request right away instead of
letting the client carry on. We do this by checking each segment's
body against the etag in the GET response's headers, like we tell
clients to do.

To make sure that the etag is the MD5, we add ?multipart-manifest=get
to all segment GET requests. That way, a segment won't be another
large object, because a large object's etag isn't its MD5. This does
mean that clients can't make, say, a DLO manifest that points to a
bunch of SLO manifests. However, that capability was only introduced
in Swift 1.13.0 (post-Havana); prior to that, clients couldn't mix
large-object types. I'm more than happy to trade that capability away
in exchange for data-integrity checks.

Change-Id: I98c78a291ee1a863a36d54a22867bd01c126644e
Merge "Check object segment MD5s"