RFR (S) 8217343: Shenandoah control thread should be able to run at critical priority

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8217343

Fix:
  http://cr.openjdk.java.net/~shade/8217343/webrev.01/

This allows for elevated thread priority for Shenandoah control thread. The change depends on
JDK-8217378 fixed first.

Testing: hotspot_gc_shenandoah, new test

Thanks,
-Aleksey