RFR: 2019-09-24, bulk backports to sh/jdk11

[Aleksey Shipilev]

Let me backport these JVMTI fixes to sh/jdk11:
  [backport] 8231197: Shenandoah: JVMTI heap walking cleanup crashes with NULL forwardee
  [backport] 8231198: Shenandoah: heap walking should visit all roots most of the time
  [backport] 8231244: Shenandoah: all-roots heap walking misses some weak roots
  [backport] 8231249: Shenandoah: GC retries are too aggressive for tests that expect OOME

Webrev:
  https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk11-20190924/webrev.01/

Except the last patch in the queue, other patches have been tested in jdk/jdk over the weekend with
no detected trouble. I would wait for next nightlies to verify the last patch too, and I don't
foresee problems.

Testing: hotspot_gc_shenandoah; vmTestbase_nsk_jvmti

-- 
Thanks,
-Aleksey