[services] Backup - Use terminateCallback for create new backup reactor

Summary:
Depends on D3560

Use termination callback (added in the previous diff) in CreateNewBackupReactor.

Test Plan:

cd services
yarn run-backup-service

Reviewers: palys-swm, geekbrother, jimpo, varun

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3561