objectpool: Count normal references when collecting stats

When fetching into object pools, we're collecting and logging statistics about the references the repository hosts. While nice, those staticstics currently don't help to derive information about the ratio of "normal" references to dangling references.

Given that we currently have some scaling problems in those areas, let's also expose the set of normal references in the object pool to allow us to see where those scalability issues come from.

Part of #1900 (closed)