azure: Specialize get_batch() to fetch objects concurrently

The default implementation is a naive 'for' loop calling 'get' sequentially.

This replaces this default implementation with a local asyncio loop, which manages a task for each of the blobs to fetch.

This reduces my cooker test run time by about 30%

Depends on !77 (closed)

Test Plan

bleh

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Migrated from D5743 (view on Phabricator)