RFR: 8249004: Reduce ThreadsListHandle overhead in relation to direct handshakes [v6]

[Daniel D.Daugherty]

> A fix to reduce ThreadsListHandle overhead in relation to handshakes and
> we add sanity checks for ThreadsListHandles higher in the call stack.
> 
> This fix was tested with Mach5 Tier[1-8]; Tier8 is still running.

Daniel D. Daugherty has updated the pull request incrementally with one additional commit since the last revision:

  8249004.cr1.patch

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4677/files
  - new: https://git.openjdk.java.net/jdk/pull/4677/files/a214b284..4e207e13

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4677&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4677&range=04-05

  Stats: 83 lines in 5 files changed: 65 ins; 2 del; 16 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