RFR: Region sampling should lock while gathering region data

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/sampling-lock/webrev.01/

Another corner case discovered when testing humongous moves: when Full GC is in full swing, heap
region states might be inconsistent. It fails the asserts in get_live() that searches for humongous
start, and cannot find it when Full GC moves humongous. Full GC does lock the heap when changing
heap region states, and thus we can lock the heap sampler too, to always get consistent view of the
heap.

Testing: hotspot_gc_shenandoah, Visualizer runs

Thanks,
-Aleksey