RFR: Bind WeakProcessor properly: simpler closures, asserts, parallelism

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/weak-processor-bind/webrev.01/

This patch addresses the leftover issues in WeakProcessor handling with
Shenandoah. It depends on bitmaps refactoring, because it uses new asserts.

Testing: tier3_gc_shenandoah (fastdebug), tier1_gc_shenandoah (release)

Thanks,
-Aleksey