RFR: Backport recent changes to shenandoah/jdk11

[Roman Kennke]

This backports a whole bunch of recent-ish Shenandoah changes from
jdk/jdk to shenandoah/jdk11. Notably it includes:

- Bugfixes&general improvements
- remaining LRB stuff
- Elimination of fwdptr
- x86_32bit
- JFR Events for Shenandoah
- Roots refactorings (fallouts from conc-class-unloading)

List of included changesets:

http://cr.openjdk.java.net/~rkennke/backport-jdk11-2019-07-31/changes.txt

Full webrev:
http://cr.openjdk.java.net/~rkennke/backport-jdk11-2019-07-31/webrev.00/

I have all changesets individually here, let me know if you see them
one-by-one.

Testing: hotspot_gc_shenandoah (x86_64) so far. Will run some more stuff
while you review :-)

This thing warrants a comb over the overall patch vs. upstream to catch
any leftovers that we don't need anymore due to LRB or nofwdptr. Let's
do that after this one's in.

Roman