RFR: 8311981: Test gc/stringdedup/TestStringDeduplicationAgeThreshold.java#ZGenerational timed out

[David Holmes]

Please see the JBS issue for full details on the underlying deadlock issue (credit to @stefank for discovering it) and the proposed solution (credit @pchilano and @xmas92 ). Quite simply we make `HandshakeState::has_operation()` non-blocking by using a `try_lock` and conservatively return `true` to indicate an operation may be pending. By not blocking we avoid the deadlock scenario. All usages of the changed code have been examined to see that they are safe with this change (they all basically just take a safe slow path to see if there really is an operation).

Testing:
- tiers 1-4, 7
- the failing string dedup test was run under our tier7 conditions, 10 times on linux-x64-debug and windows-x64-debug

Given the nature of the deadlock this testing is not sufficient to claims success as we probably only saw 1 failure in many hundreds of runs. So if anyone has suggestions for additional testing please speak up. Otherwise we are relying on "correctness by design" - we've removed a blocking condition that leads to the 3-way deadlock, and examined the code paths affected.

Thanks.

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Commit messages:
 - Merge branch 'master' into 8311981-handshake-deadlock
 - 8311981: Test gc/stringdedup/TestStringDeduplicationAgeThreshold.java#ZGenerational timed out

Changes: https://git.openjdk.org/jdk/pull/15240/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15240&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311981
  Stats: 11 lines in 1 file changed: 9 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15240.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15240/head:pull/15240

PR: https://git.openjdk.org/jdk/pull/15240