[services] Backup - Add server reactor implementations - recover backup key reactor

Summary:
Depends on D3516

Add implementation for the recover backup key reactor

Test Plan:

cd services
yarn run-backup-service

Reviewers: palys-swm, geekbrother, varun, jimpo

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3518