[services] Backup - Add client reactor base classes - write reactor

Summary:
Depends on D3514

Adding client base classes for reactors - write reactor

Test Plan:

cd services
yarn run-backup-service

Reviewers: jimpo, geekbrother, palys-swm, varun

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3515