[services] Backup - Update service implementation

Summary:
Depends on D3518

Use server reactor implementations from the D3467 in the service implementation

Test Plan:

cd services
yarn run-backup-service

Reviewers: jimpo, geekbrother, varun, palys-swm

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3468