[services] Backup - Fix primary key logic

Summary:
Based on my research about the dynamo DB: https://phabricator.ashoat.com/D2950#81654

• Every DynamoDB table has a unique primary key
• A primary key [PKSK] must be composed of a partition key [PK], and can optionally have a sort key [SK].
• A GetItem request returns one and only one item using its unique primary key [PKSK].
• A Query does a fast lookup and must specify one and only one partition key [PK]. It can return multiple items.
• A Scan evaluates every item in a table and may return a subset based on filter parameters. Scans are the correct choice in some circumstances but can be slow and costly if used incorrectly.

Depends on D3176

Test Plan: Backup service still builds

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

Reviewed By: palys-swm, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3177