RFR: 8319137: release _object in ObjectMonitor dtor to avoid races [v3]

[Patricio Chilano Mateo]

> Please review the following fix which moves the release of the _object WeakHandle back to the ObjectMonitor dtor. Releasing it earlier as we do today introduces a race which can lead to crashes. Tested running tiers123 in mach5.
> 
> This fix is an effective backout of [JDK-8256302](https://bugs.openjdk.org/browse/JDK-8256302).
> 
> Thanks,
> Patricio

Patricio Chilano Mateo has updated the pull request incrementally with one additional commit since the last revision:

  remove redundant branches

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/16738/files
  - new: https://git.openjdk.org/jdk/pull/16738/files/3cfea97d..544bc347

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16738&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16738&range=01-02

  Stats: 6 lines in 1 file changed: 0 ins; 6 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16738.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16738/head:pull/16738

PR: https://git.openjdk.org/jdk/pull/16738