RFR: Bulk backports jdk12 -> sh/jdk11

[Roman Kennke]

This backports all relevant Shenandoah changes (I hope) from 
jdk-updates/jdk12u to shenandoah/jdk11.

They all applied mostly clean and trivial. Builds and tests look good.

List of changes:
http://cr.openjdk.java.net/~rkennke/upstream-jdk11-merge-2019-03-11/changes.txt

Full webrev:
http://cr.openjdk.java.net/~rkennke/upstream-jdk11-merge-2019-03-11/webrev.00/

Ok?

Roman