RFR: Backport JDK-8221435 and JDK-8221629

[Roman Kennke]

This backports the two changes:

- JDK-8221435: Shenandoah should not mark through weak roots
- JDK-8221629: Shenandoah: Cleanup class unloading logic

to shenandoah/jdk11

They are a fairly tricky beast, that's why I post them separately. They
should be functionally and structurally equivalent to what's in jdk/jdk,
but required some changes because weakProcessor is entirely different in
jdk11, and not as capable (no parallel processing). Zhengyu and I
eyeballed that it's doing the right thing (i.e. not mark through weak
roots).

Testing: hotspot_gc_shenandoah and eyeballing

Webrev:
http://cr.openjdk.java.net/~rkennke/backport-jdk11-2019-05-15/webrev.00/

Good?

Roman