[services] Reactors - use doneCallback only from OnDone

Summary:
Depends on D3488
We want to use useDoneCallback only from OnDone because OnDone is executed when there are no more grpc operations to perform and we should have access to that if we'd like to release some resources, etc.

Test Plan:

cd services
yarn run-backup-service

Reviewers: geekbrother, jimpo, palys-swm, varun

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3529