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
Migrated from D5743 (view on Phabricator)