[services] Backup - Fix copying big data chunk

Summary:
Depends on D3471

Once we receive data as a gRPC field, we don't want to copy it around, especially when it's a data chunk and the size of it may reach up to 4-5MB. Instead, it's always better to have just one place in memory for it and pass the address around.

Test Plan:

cd services
yarn run-backup-service

Reviewers: geekbrother, varun, palys-swm, jimpo, ashoat, marcinwasowicz

Reviewed By: palys-swm, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3487