RFR: 8297638: Memory leak in case of many started-dead threads [v2]

[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.

Chris Plummer has updated the pull request incrementally with one additional commit since the last revision:

  Cleaned up removeUnreachableListeners().

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12081/files
  - new: https://git.openjdk.org/jdk/pull/12081/files/894a33d6..f2acfacc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12081&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12081&range=00-01

  Stats: 16 lines in 1 file changed: 5 ins; 0 del; 11 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