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

[Roman Kennke]

This merges in tags jdk-12+3 and jdk-12+4, and adds corresponding
shenandoah-jdk-12+3 and shenandoah-jdk-12+4 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/jdk-upstream-merge-2018-07-27/outgoint.txt

Ok?
Roman