[services] Backup - Send log - add terminate callback

Summary:
Depends on D3649

Adding a terminate callback so we wait until the chunks upload is finished (if it ever should be performed in a current case).

Test Plan:

cd services
yarn run-backup-service

Reviewers: palys-swm, geekbrother

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3650