[services] Backup/Blob - Make sure Finish is only called once

Summary:
Depends on D3636

We should make 100% sure that Finish (the method provided by gRPC) is called no more than once. Otherwise, it's going to crash.

Test Plan:

cd services
yarn run-blob-service
yarn run-backup-service

Reviewers: palys-swm, geekbrother, ashoat

Reviewed By: palys-swm, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3664