RFR: Multiple minor fixes

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/multi-1/webrev.01/

These are minor fixes that were sitting and rotting in my patch queue:

 *) The indenting in mark-compact code drives me nuts;
 *) Full GC should acquire the heap lock once when starting to deal with regions;
 *) Print "process weakrefs", to avoid confusion with coalesced UR;
 *) Micro-optimize get_region

Testing: tier3_gc_shenandoah

Thanks,
-Aleksey