R1:1b0ef4391032

Separate internal and external Query Cursors more cleanly, to fix pagination against broken objects

Summary:
Ref T13259.

(NOTE) This is "infrastructure/guts" only and breaks some stuff in Query subclasses. I'll fix that stuff in a followup, it's just going to be a larger diff that's mostly mechanical.

When a user clicks "Next Page" on a tasks view and gets `?after=100`, we want to show them the next 100 //visible// tasks. It's possible that tasks 1-100 are visible, but tasks 101-788 are not, and the…