[keyserver] Delete a holder after deleting a blob

Summary:
When deleting an invite link, check what was its blob holder, and schedule its deletion.

Depends on D9968

Test Plan: Delete a link and check if a holder is deleted in the Blob service. Also tested if deleting a link with an invalid holder doesn't cause any issues.

Reviewers: michal, bartek, rohan

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9972