[services-lib] Add DDB batch operation helpers

Summary:
We use DDB batch operations in several places: reserved usernames write, saving new one-time keys, blob cleanup (soon), reports storage.

DDB BatchWriteItem operation API can be cumbersome - 25 items limit, unprocessed items, throttling etc. Their docs also recommend exponential backoff strategy. I decided to create an API that handles all this overhead for us. See also comments in D8452.

This diff adds some exponential backoff helpers. See child diff for actual batch_write impl.

Test Plan: Tested together with child diff

Reviewers: varun, michal, jon

Reviewed By: varun, michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8894