RFR: 8249004: Reduce ThreadsListHandle overhead in relation to direct handshakes [v8]

[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:

  coleenp CR - add comments to clarify what WB_HandshakeReadMonitors() and WB_HandshakeWalkStack() expect.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4677/files
  - new: https://git.openjdk.java.net/jdk/pull/4677/files/48416864..9518a9a8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4677&range=07
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4677&range=06-07

  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 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