RFR: 8249004: Reduce ThreadListHandle overhead in relation to direct handshakes

[Daniel D.Daugherty]

A trivial fix to reduce ThreadListHandle overhead in relation to handshakes.

This refactoring was tested with Mach5 Tier[1-3].

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Commit messages:
 - 8249004: Reduce ThreadListHandle overhead in relation to direct handshakes

Changes: https://git.openjdk.java.net/jdk/pull/4677/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4677&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8249004
  Stats: 10 lines in 3 files changed: 7 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4677.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4677/head:pull/4677

PR: https://git.openjdk.java.net/jdk/pull/4677