RFR (S): Add the option to disable Shenandoah SATB barriers

[Aleksey Shipilev]

We thought it is folded under ShenandoahWriteBarrier, and some of it is.
However, some paths enable SATB unconditionally, which distorts our differential
performance runs.

Introducing ShenandoahSATBBarriers option:
  http://cr.openjdk.java.net/~shade/shenandoah/satb-barriers-disable/webrev.02/

Testing: hotspot_gc_shenandoah, benchmarks

Thanks,
-Aleksey