[sgen] Run the scan pinned object job concurrently with minors

[sgen] Run the scan pinned object job concurrently with minors

Concurrent mark doesn't normally scan objects in the nursery since they can be moved. At the end of the concurrent mark, we were locking over the entire gc pause in order to make sure that the we can scan the latest pinned object list (aka none of those objects can be moved). Lock only over the pinning phase of the collection, since it is enough.