Merge pull request #2803 from BrzVlad/feature-conc-pinned-scan

[sgen] Scan pinned objects in nursery as part of concurrent mark

Objects pinned in the nursery can hide behind them a large graph of objects in the heap which is never scanned because we don't follow references in the nursery. After precleaning, we scan the latest set of pinned objects (while locking in order to make sure that the set doesn't change, so none are moved).

diff --cc mono/sgen/sgen-gc.c
Simple merge

diff --cc mono/sgen/sgen-pinning.c
Simple merge

diff --cc mono/sgen/sgen-pinning.h
Simple merge