Review GetResult and LookupInResult design around edge cases

When you do a collection.get(<key>, GetOptions(with_expiry=True)), you expect that there will be a property on the GetResult that is returned with the expiry, and when you do result.content_as[dict] you will see the entire document (and nothing else). If you instead ask for a projection collection.get(<key>, GetOption(project=["a", "b"], with_expiry=True)), you'd expect the expiry like before, and the content_as to give you just the projection.

In fact, the former (thanks to a recent hack) is what happens. But the latter case gives you the projection, plus the expiry field from the xattrs ("$document.exptime: xxx")

In fact this is a bit tricky under the covers. The GetResult realized it was created from a SubdocResult, and needs to do a bit of work to get what you'd expect for a GetResult. Perhaps we can rethink what I did? Doing things close to GA, I did the most expedient thing I could do, and probably with a bit of thought, maybe a somewhat more clever ContentProxy, we can clean this up, and eliminate the xattr field from the result of a content_as.

Also... should we be more clever about how we return results to show which are xattrs? Worth pondering.