Summary:
Original commit: 0e6d59b448ad954fd8d00846e17c831350a4f195 / D42592
Improved Azure blob delete performance by moving to the batch client. This will
allow us to delete all blobs related to a single backup with one API call. We
saw 30%+ performance improvements on small backups, and expect that to grow
for a normal sized backup.

In addition, we can now delete backups in parallel, to help improve the backup
deletion performance even more.

Test Plan:
ran some tests comparing the 2 deletion implementations
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small (30mb)
-----
single | batch
2721   | 2655
2752   | 2561
2793   | 2509
2779   | 2525
2792   | 2559

--------
Bigger (390mb)
-------
single | batch
3366   | 2611
3466   | 2805
4027   | 2872
3327   | 2611
3422   | 2740
---
in addition, tested backup deletion workflow via UI

Reviewers: anijhawan, vkumar, nsingh, daniel

Reviewed By: anijhawan, daniel

Differential Revision: https://phorge.dev.yugabyte.com/D42636