RFR: (sh/jdk11): Bulk backports 2020-03-03

[Roman Kennke]

This backports the following outstanding changes from jdk/jdk to
shenandoah/jdk11:

JDK-8230350: Shenandoah: Assertion failed when GC is cancelled by a
worker thread
JDK-8232747: Shenandoah: Concurrent GC should deactivate SATB before
processing weak roots
JDK-8233401: Shenandoah: Refactor/cleanup Shenandoah load barrier code
JDK-8233339: Shenandoah: Centralize load barrier decisions into
ShenandoahBarrierSet
JDK-8237543: Shenandoah: More asserts around code roots
JDK-8237776: Shenandoah: Wrong result with Lucene test
JDK-8238574: Shenandoah: Assertion failure due to missing null check
JDK-8239354: Shenandoah: minor enhancements to traversal GC

Some changes are only fragments of the original change. For example,
JDK-8233401 leaves out the native-barrier parts because they are not
present in shenandoah/jdk11. Parts of original JDK-8233339 have already
arrived via other backport, etc. Please review carefully.

Changesets:
http://cr.openjdk.java.net/~rkennke/backport-shjdk11-2020-03-03/changesets.txt
Webrev:
http://cr.openjdk.java.net/~rkennke/backport-shjdk11-2020-03-03/webrev.00/

Testing: hotspot_gc_shenandoah (x86_64 and aarch64)

Thanks,
Roman