Final diff for ENG-9368.

• Besides Tunnelbroker and Backup data, blob holders are also removed
• Some renames

Depends on D13651

Mocked this by creating a device by Commtest, manually added some holders prefixed by its device ID, then logging it out (also with commtest).
Checked Identity logs and DDB by querying for indexed_tag=deviceID. Verified that holders were gone.

Final testing will be done on staging:

• Log in a device, jot down its device ID, start a DM thread, set avatar to image, then reset back to emoji.
• Simulate blob service connection error - change blob URL client to fake, or comment-out the call and return failure.
• Log out the device.
• Verify Identity logs and DDB by querying for indexed_tag=deviceID. Holders should be gone.