[services] Blob - Add holder to the blob DB if data exists

Summary:
Depends on D3801

If the data already exists in the blob, we just want to add a new holder pointing to an existing hash.

Test Plan: Like in D3535 and hardcode fixed data hash in the client code, upload twice to the blob and see if it works as expected: it should upload the data only once and add both blobs pointing to the same hash to the database

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: ashoat, palys-swm, Adrian, atul, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3814