RFR (S) 8225229: Shenandoah: trim down default number of GC threads

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8225229

We can trim down the default number of GC threads to make Shenandoah more cooperative with the
application and OS scheduler. We routinely trim down concurrent threads when deploying, and better
default would make deployment stories simpler. Various tests show that defaulting to 1/4*N_CPU
concurrent and 1/2*N_CPU parallel threads hits the sweet spot. New test verifies boundary conditions.

Patch:
  http://cr.openjdk.java.net/~shade/8225229/webrev.01/

Testing: hotspot_gc_shenandoah {fastdebug|release}

-- 
Thanks,
-Aleksey