Request for review(XS): 6819098 G1: reduce RSet scanning times

[Igor Veresov]

Modified previous naive work stealing algorithm by introducing a minimal 
work unit for the helper threads participating in the RSet scan (thus 
avoiding competition for each card), also added bounded exponential 
skipping.

Testing: JBB2005 on a 16-core intel core2 box with 30G heap (25G young 
gen), 13 GC threads. The RSet scanning times reduced ~600%.

Webrev: http://cr.openjdk.java.net/~iveresov/6819098/webrev.00