RFR: Upstream merge jdk/jdk11 -> shenandoah/jdk11 2018-07-27

[Roman Kennke]

This merges in tags jdk-11+23 and jdk-11+24, and adds corresponding
shenandoah-jdk-11+23 and shenandoah-jdk-11+24 tags. The same 'precise'
merging+tagging procedure as last time was used.

Only minor/trival merge conflicts arose and no significant (to
Shenandoah) new features arrived.

List of stuff merged in:
http://cr.openjdk.java.net/~rkennke/jdk11-upstream-merge-2018-07-27/outgoint.txt

It did not regress testing (locally at least)

ok?

Roman