[services] Backup - Pull more mutual code from base reactors - Add base reactor

Summary:
Depends on D3873

https://linear.app/comm/issue/ENG-916/consider-pulling-more-mutual-code-from-the-async-reactors

Adding a master base reactor for all reactors

Test Plan:

cd services
yarn run-backup-service

Reviewers: palys-swm, ashoat

Reviewed By: palys-swm, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3785