Expose "key" for each record

[jono-allen]

We are using graphql on top of eletrodb which has been working great, we are having some issues regarding relay pagination.

The relay spec requires a cursor for each record and an endCursor for each edge.
Eletrodb's cursor works for the endCursor but there is no way to get the cursor for each record.

Our workaround for this is to manually re-create the cursor for each record returned from the query by providing data: 'includeKeys' and then constructing a cursor for the relay to match the pattern of the Eletrodbs cursor.

The issue arises when the query doesn't use the primary index, and we then need to provide other keys to the cursor but it's not easy to work out which other index key to provide.

It would be great if eletrodb had an option to expose a key for each record that matches the eletrodbs cursor key for the current index being queried.
This key then could be provided as the cursor for the relay edge.