Improve nursery pining performance by making sure scan starts are filled.

Improve nursery pining performance by making sure scan starts are filled.

	* sgen-gc.c (set_nursery_scan_start): Only set scan start if the
	new value is smaller than the previous one. This check is racy, but
	it's a benign one.

	* sgen-gc.c (mono_gc_try_alloc_obj_nolock): Try harder filling
	scan starts. With this change we go from a 3-20% fill rate to
	over 90% on average.

	Pinning performance increased 6x on an extreme benchmark.