[services][blob] Implement Delete blob HTTP handler

Summary:
Resolves ENG-3524

THis diff implements the DELETE /blob/{holder} endpoint which revokes a blob holder. If last holder is revoked, the blob is deleted. This behavior is the same as for Remove blob RPC.

Depends on D7465

Test Plan:
Created a blob with a few holders and called this endpoint to remove them one by one.
Calling GET before returns HTTP 200, after deleting returns HTTP 404 as expected.

Reviewers: marcin, tomek, jon, michal

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7466