RFR: 8297638: Memory leak in case of many started-dead threads

[Chris Plummer]

Fix JDI leak when the debuggee creates a lot of threads, while at the same the debugger is not sending any commands. The lack of commands being sent results in code not being triggered that normally would clear out unreachable listeners and also clear out ObjectReferences queued for disposal.

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Commit messages:
 - Fix thread memory leak

Changes: https://git.openjdk.org/jdk/pull/12081/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12081&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297638
  Stats: 194 lines in 3 files changed: 192 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12081.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12081/head:pull/12081

PR: https://git.openjdk.org/jdk/pull/12081