RFR: 8320304: Refactor and simplify monitor deflation functions [v5]

[Stefan Karlsson]

> The recent rewrites to remove safepointed monitor deflation allows us to simplify the monitor deflation code a bit. The code is now guaranteed to be called from the MonitorDeflationThread, which is a JavaThread, and never from the VMThread.
> 
> I would like to propose two changes to the code:
> 1) Remove all code that supports monitor deflation from other threads than the MonitorDeflationThread
> 2) Extract out the logging code so that it is easier to see the structure of the monitor deflation code
> 
> I have run minimal manual of this patch. I'd like to get some feedback on the proposal before I proceed to run more in-depth testing.

Stefan Karlsson has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:

 - Merge remote-tracking branch 'upstream/master' into 8320304_cleanup_of_monitor_deflation
 - David's review comments
 - Dan's review comments
 - Make classes StackObjs
 - 8320304: Refactor and simplify monitor deflation functions

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Changes: https://git.openjdk.org/jdk/pull/16706/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16706&range=04
  Stats: 225 lines in 2 files changed: 102 ins; 71 del; 52 mod
  Patch: https://git.openjdk.org/jdk/pull/16706.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16706/head:pull/16706

PR: https://git.openjdk.org/jdk/pull/16706