RFR: 8318757 ObjectMonitor::deflate_monitor fails "assert(prev == old_value) failed: unexpected prev owner=0x0000000000000002, expected=0x0000000000000000"

[Stefan Karlsson]

A safepointed monitor deflation pass can run interleaved with a paused async monitor deflation pass. The code is not written to handle that situation and asserts when it finds a DEFLATER_MARKER in the owner field. @pchilano also found other issues with having to monitor deflation passes interleaved. More info below ...

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Commit messages:
 - Add patricio's test
 - Rework ObjectMonitorsHashtable for thread dumping

Changes: https://git.openjdk.org/jdk/pull/16519/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16519&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318757
  Stats: 429 lines in 9 files changed: 216 ins; 147 del; 66 mod
  Patch: https://git.openjdk.org/jdk/pull/16519.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16519/head:pull/16519

PR: https://git.openjdk.org/jdk/pull/16519