Don't allow listing indexes via the `retrieveDocument` API

We would need to:

- [ ] Prevent new documents from having the `_id` `_index`, either by rejecting those documents, or mapping the string `_index` to another internally managed string. Affected apis: `createDocument` `bulkDocuments`
- [ ] when a `retrieveDocument` request arrives, either reject the lookup for `_index` or map the string similar to aforementioned mapping
- [ ] **IF** the mapping approach is taken, the document's `_id` would need to be reverse mapped back to the string `_index` on any retrievals. Affected apis: `retrieveDocument`, `listDocuments`, `queryDocuments`
- [ ] add a test case for each

The easier of the two is just to reject any documents with an `_id` set to `_index`, but is arguably leaking impl details to the consumer.

## Background

Related Issue: https://github.com/hyper63/hyper/issues/486

So something I discovered today that probably should not be allowed?

If I have a hyper data service, I can retrieve all of the indexes that exist on the database by calling `GET /data/default/_index`. This gets handled by the "get document by id" route, which calls `retrieveDocument` on the adapter.

Ultimately the CouchDB adapter does:

```js
asyncFetch(`${config.origin}/${db}/${id}`,...)
```

with `_index`as the id.

Of course, there isn't a document with _id _index, BUT that does map to the route on the CouchDB API that lists all of the indexes for the DB: https://docs.couchdb.org/en/stable/api/database/find.html#get--db-_index. And since the data port says anything can be returned from the `retrieveDocument` API, hyper simply returns that response and you get something like:

```json
{
"total_rows": 1,
"indexes": [
{
"ddoc": null,
"name": "_all_docs",
"type": "special",
"def": {
"fields": [
{
"_id": "asc"
}
]
}
}
]
}
```