Simplify Modeled Message Marshalling

In v1 API, most request/response model objects were Serializable, so it was easy to persist those in all kinds of client side caches. In particular, s3.model.ObjectMetadata could be persisted as is in client-side s3 object cache (and those are invaluable when dealing with big buckets over high latency links).

In v2, unfortunately, s3.model.HeadObjectResponse is not Serializable, and thus can not be persisted as is - instead, manual copying of fields or reflection/security tricks are required.