RFR: 8320450: Move MonitorDeflationThread inside ObjectSynchronizer

[David Holmes]

At this point it is more like "move as much async deflation control code into MonitorDeflationThread from ObjectSynchronizer as possible". The next step would be to look at making MDT a nested class (but keep the seperate cpp file).

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Commit messages:
 - Commit 2: Move as much of the async deflation control code inside MDT.
 - Commit 1: make everything we can private and declare DMT as a friend

Changes: https://git.openjdk.org/jdk/pull/22998/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22998&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320450
  Stats: 408 lines in 5 files changed: 216 ins; 175 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/22998.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22998/head:pull/22998

PR: https://git.openjdk.org/jdk/pull/22998