Properly handle custom primary keys for is_scatter_gather
Properly handle custom primary keys for is_scatter_gather

Summary: mistakenly classifying a query as scatter gather when the primary key was custom

Test Plan: ran locally

Reviewers: adam, david, bpalsson, rli

Differential Revision: https://phab.wish.com/D41957

ContextLogic/mongoengine,ContextLogic/mongoengine