Support for a callback on slow queries

Summary:
If a query takes more than 100ms and a callback has been set, call that
with info about the query. This'll let us log out slow queries to aid in
prod perf debugging.

Test Plan: Unit tests for safety; used with D9905 for correctness

Reviewers: jack, david, yanwei, josh

Maniphest Tasks: T5825

Differential Revision: http://infra1.corp.contextlogic.com/D9906