RFR: 8265028: JDWP debug agent thread lookup can be made faster

[Chris Plummer]

Improved thread lookup speeds in the debug agent, especially when there is a large number of threads. See CR for details.

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Commit messages:
 - Faster debug agent thread lookups

Changes: https://git.openjdk.java.net/jdk/pull/3444/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3444&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265028
  Stats: 30 lines in 1 file changed: 8 ins; 10 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3444.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3444/head:pull/3444

PR: https://git.openjdk.java.net/jdk/pull/3444