Improve pagination performance

[matthew-white]

Filing this as a separate issue from #1099, which is about documentation. This issue is to improve the performance of paging submissions and entities. As noted in the API docs, "paging … will not greatly improve the performance of exporting data" relative to exporting all data at once.

This issue also relates to #1074. As @yanokwa asks at #1099 (comment):

paging does not greatly improve performance

How big of a lift is it to resolve this? It'd be nice to offer a carrot when we change defaults from returning all submissions.

[matthew-white]

@brontolosone writes at #1099 (comment):

instead of using LIMIT + OFFSET, we could consider doing pagination using window functions.