use dispatch on main queue

Summary: Ths differential overwrites solution from its parent diff. Instead of waiting to call completionHandler until rescinding loop completes, we schedult its call on main thread.

Test Plan: Test plan is the same as for its parent diff

Reviewers: palys-swm, atul, ashoat

Reviewed By: ashoat

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4576