Initial DataSource setup

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

desktop/.eslintrc.js

@@ -109,6 +109,7 @@ module.exports = {
         // for reference: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/README.md#extension-rules
         'no-unused-vars': 0,
         'no-redeclare': 0,
+        'no-dupe-class-members': 0,
         '@typescript-eslint/no-redeclare': 1,
         '@typescript-eslint/no-unused-vars': [
           1,