Online GC: Implement race condition safeguards

Problem to solve

As documented in the specification, the third and last stage of online GC is to identify and remove dangling blobs and manifests.

During this stage, several race conditions may occur between the garbage collector and the API. These were identified and measures were designed and documented in the spec to safeguard us from such conditions.

Proposal

Implement race condition safeguards as described in the spec.