[services] Backup - Implementation update

Summary:
Update the implementation so it uses the base reactor from D3246.

Also, we edit the proto file so we can use the base reactor for one-direction streams.

Depends on D3246

Test Plan:

cd services
yarn run-backup-service

Reviewers: palys-swm, geekbrother, varun, jimpo, atul

Reviewed By: palys-swm, varun, atul

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

Differential Revision: https://phabricator.ashoat.com/D3248