RFR: Added logging for the number of workers used for GC cycles

[Zhengyu Gu]

Added logging for the number of workers used for GC cycles.

Also fixed a few of calls for calculating concurrent workers to use 
right AdaptiveSizePolicy API. However, this is just for correctness, 
there is not fundamental difference, since Shenandoah always setup 
parallel # = conc # (when parallel # > 1), and AdaptiveSizePolicy 
basically uses the same algorithm to calculate parallel # and conc #.

We should also look into to tune the sizing settings/algorithm, it looks 
to me that heap capacity factor can easily dominate result. By default, 
it uses 1 worker for 32M heap space, which only takes 256M heap space to 
max out 8 parallel/concurrent workers. Given the heap size that 
Shenandoah is targeting, it is almost always going to max out 
parallel/conc workers.


http://cr.openjdk.java.net/~zgu/shenandoah/calc_conc_workers/webrev/

Test:

  tier3_gc_shenandoah.


Thanks,

-Zhengyu