implement faster pagination

[geospatial-jeff]

We switched to keyset pagination using the sqlakeyset library because keyset pagination is faster than limit/offset, especially at higher page counts where limit/offset has to scan through large portions of the table.

The paging is still very slow though, I think because I've only gotten sqlakeyset to work with a two column sort which is reportedly very slow with postgres. So there could be a few things going on:

1. User error on my part, and sqlakeyset can work with a single column sort and be really fast.
2. Sqlakeyset requires a two column sort and we need to rethink our paging implementation.

Regardless of the solution it would be nice for paging to be faster.