Implement postgres keyset pagination

[reshmakh]

We currently use "OFFSET"/"LIMIT" for pagination. It's simple and works great for our current data size.

As we get to larger data sets, it will start to incur some performance penalties.

The "right" way to implement pagination at scale is called "keyset" pagination:
https://www.citusdata.com/blog/2016/03/30/five-ways-to-paginate/
https://leopard.in.ua/2014/10/11/postgresql-paginattion

This is supported/encouraged in the FHIR graphql spec:
https://www.hl7.org/fhir/graphql.html#searching

(named "cursor", unrelated to a db cursor)