R1:3ec4984f27cd

Use cursor-based paging in Differential

Summary:
Ref T603. Ref T2625. Use cursors to page Differential queries, not offsets.

The trick here is that some queries are ordered. In these cases, we either need to pass some kind of tuple or do a cursor lookup. For example, if you are viewing revisions ordered by `dateModified`, we can either have the next page be something like:

?afterDateModified=2398329373&afterID=292&order=modified

...or some magical token:

?afterToken=2398329373:292&order=modified…