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
@czue Curious why we don't use couchdb show functions in other places like `case/get_lite` view?