add document show function for doc_type, date and domain

Useful if all you want is a doc's domain / doc_type / date. See [pillowtop retry](https://github.com/dimagi/pillowtop/pull/89/files#diff-6dac4be25b780fb79c406285662331fcR300) for an example use.

REQUIRES REINDEX
I see a couple calls like `cls.get_db().view("case/get_lite", keys=ids, ...`, which gets a bunch of cases in one request. I think you'd have to do one request per case with a show. Sounds plausible that that tradeoff is worth it, but something to think about