RFR: Protect more internal code from false sharing

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/paddings/webrev.01/

This makes more internal code resilient to false sharing. I have inspected all (?) places where we
do multi-threaded access to contended data, and put the paddings around them. There is the
ShenandoahSharedCounter abstraction in work, but it requires quite a bit more attention to get right.

This seems to improve final-mark performance on SPECjbb due to code root scanning improvements. I'll
run a few other benchmarks too overnight.

Testing: tier3_gc_shenandoah

Thanks,
-Aleksey