Map XFormInstance doc type to XFormInstanceSQL
Map XFormInstance doc type to XFormInstanceSQL

The other side of this is `get_document_store_for_doc_type`, which uses `XFormInstanceSQL.ALL_DOC_TYPES` to look up the form document store.

dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq