RFR: 8249004: Reduce ThreadsListHandle overhead in relation to direct handshakes [v9]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 10 additional commits since the last revision:

 - Merge branch 'master' into JDK-8249004
 - coleenp CR - add comments to clarify what WB_HandshakeReadMonitors() and WB_HandshakeWalkStack() expect.
 - dholmes CR - change NULL to nullptr.
 - 8249004.cr1.patch
 - Merge branch 'master' into JDK-8249004
 - Merge branch 'master' into JDK-8249004
 - Merge branch 'master' into JDK-8249004
 - Merge branch 'master' into JDK-8249004
 - Merge branch 'master' into JDK-8249004
 - 8249004: Reduce ThreadListHandle overhead in relation to direct handshakes

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4677/files
  - new: https://git.openjdk.java.net/jdk/pull/4677/files/9518a9a8..045b3e0d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4677&range=08
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4677&range=07-08

  Stats: 44007 lines in 1055 files changed: 33358 ins; 6622 del; 4027 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