[blob-service] Implement holder management with new db

Summary:
Part of ENG-4269.

Mostly copied logic from http::handlers::blob::assign_holder_handler and http::handlers::blob::remove_holder_handler - but now with using the new database methods

Partially addresses the TODO from http::handlers::blob::remove_holder_handler - auto deletion of orphaned blobs can now be disabled

Depends on D8455.

Test Plan: Tested later in the stack

Reviewers: michal, varun, jon, patryk

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8456