RFR: 8241142: Shenandoah: should not use parallel reference processing with single GC thread

[Aditya Mandaleeka]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8241142

Webrev:
  https://cr.openjdk.java.net/~adityam/8241142/webrev.00/

This change makes it so that MT reference processing is no longer enabled when there is only a
single GC thread.

hotspot_gc_shenandoah passes with -XX:ParallelGCThreads=1

Thanks,
Aditya