Summary:
For context see https://fb.workplace.com/notes/470523670998369
This diff introduces the DataSource abstraction, that can store records. If a key is set a key -> record mapping is stored, to make it easy to update existing records using `upsert`, without knowing their exact index.
Internal storage will be slightly altered in upcoming diffs, so don't pay to much attention to that part.
Reviewed By: nikoant
Differential Revision: D25953337
fbshipit-source-id: 1c3b53a2fcf61abaf061946be4af21d2aecc6c6d
0dc1abdac4