RFR (sh/jdk8): Bulk backports sh/jdk11 -> sh/jdk8

[Roman Kennke]

This backports the following changes from sh/jdk11 to sh/jdk8:

JDK-8221751: Shenandoah: Improve SATB enqueueing
JDK-8223184: Shenandoah: Missing roots in SRP::process_all_roots_slow
JDK-8223951: Shenandoah: Only need to update thread roots during final
update refs
JDK-8224179: Shenandoah: CTW test failures with traversal GC
JDK-8224495: Shenandoah: Do not rescan code roots in final mark pause if
it is not degenerated GC+
JDK-8224508: Shenandoah: Need to update thread roots in final mark for
piggyback ref update cycle
JDK-8225341: Shenandoah: CM::update_thread_roots() needs to handle
derived pointers

I am working through the outstanding backports list from oldest to
newest. This is the first batch of a few that are coming up next. I am
thinking it's better to chop it in smaller pieces and have this tested
by CI while I work on the next batch.

Some backports are only fragments (e.g. JDK-8221751) because most of it
landed via other backports (e.g. arraycopy stuff) already.

Changesets:
http://cr.openjdk.java.net/~rkennke/backport-shjdk8-2020-02-27/changesets.txt
Webrev:
http://cr.openjdk.java.net/~rkennke/backport-shjdk8-2020-02-27/webrev.00/

Testing: hotspot_gc_shenandoah

Roman