RFR: Upstream merge from jdk/jdk

[Roman Kennke]

Let's merge the first jdk13 tag!

Notable news:
- 8204947: Port ShenandoahTaskTerminator to mainline and make it default

and!

- 8214259: Implementation: JEP 189: Shenandoah: A Low-Pause-Time Garbage
Collector (Experimental)

:-D

.. and a few related commits.

http://cr.openjdk.java.net/~rkennke/upstream-jdk13-merge-2018-12-14/outgoing.txt

Testing: hotspot_gc_shenandoah

ok?

Roman