[8] 2019-10-29: Bulk backports to sh/jdk8

[Aleksey Shipilev]

https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20191029/webrev.01/

This batch brings in a few simple backports to sh/jdk8, in preparation for x86_32 backporting work
and exposing sh/jdk8 to wider testing. I carefully selected backports that do not clash with
Traversal and/or larger changesets.

Changes:
 [backport] 8223450: Disable Shenandoah C2 barriers verification for x86_32
 [backport] 8224881: Shenandoah: trashing "Collection Set, Pinned" region during Degenerated GC
 [backport] 8231946: Remove obsolete and unused ShenandoahVerifyObjectEquals flag
 [backport] 8225229: Shenandoah: trim down default number of GC threads
 [backport] 8231932: Shenandoah: conc/par GC threads ergonomics overrides user settings
 [backport] 8226957: Shenandoah: Remove obsoleted ShenandoahStoreCheck option
 [backport] 8232702: Shenandoah: gc/shenandoah/TestVerifyJCStress.java uses non-existent
-XX:+VerifyObjectEquals
 [backport] 8225017: [TESTBUG] gc/shenandoah/oom/TestThreadFailure.java takes too long

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey