RFR: 8255527: Shenandoah: Let ShenadoahGCStateResetter disable barriers [v2]

[Roman Kennke]

> JDK-8255036 introduced a regression: the new ShenandoahGCStateResetter does not actually disable barriers: it only records old state (without changing it) and then restores that state (i.e a no-op). 
> 
> Testing: hotspot_gc_shenandoah (also on conc-weakref branch, where the bug did manifest)

Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:

  Make heap local variable const for consistency

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/899/files
  - new: https://git.openjdk.java.net/jdk/pull/899/files/4515272b..e51fb430

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=899&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=899&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/899.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/899/head:pull/899

PR: https://git.openjdk.java.net/jdk/pull/899