RFR: More efficient next bitmap resetting for traversal

[Roman Kennke]

The resetting of next bitmap for traversal is a bit inefficient:
- It's iterating *all* regions, but then only picks traversal-set
regions. We can iterate traversal-set to begin with instead.
- It grabs the heap-lock for each region. Which basically serializes the
whole process for no good reason.

http://cr.openjdk.java.net/~rkennke/better-traversal-reset-bitmaps/webrev.00/

Testing: tier3_gc_shenandoah

Ok?

Roman