RFR (sh/jdk11): Pick up jdk-11.0.6+4

[Roman Kennke]

Let's pick up jdk-11.0.6+4 to shenandoah/jdk11. Merges trivially, and 
passes my local testing (hotspot_gc_shenandoah, tier1, tier2).

http://cr.openjdk.java.net/~rkennke/upstream-jdk-11.0.6%2b4/changesets.txt

I'm also tagging with shenandoah-jdk-11.0.6+4

Good?

Roman