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

Summary:
Depends on D3512

This class is moved and renamed for a better folder structure.
As we need to add client reactors in order to be able to connect to the blob service, we need to distinguish server reactors from client reactors.

Test Plan:

cd services
yarn run-backup-service

Reviewers: geekbrother, jimpo, varun, palys-swm

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3513