RFR: 8267079: Support async handshakes that can be executed by a remote thread

[Man Cao]

Hi all,

Can I have reviews for this small refactoring change? It resolves a pending concern from [JDK-8238761](https://bugs.openjdk.java.net/browse/JDK-8238761), clarifies the code and allows more use case of async handshakes. See [JDK-8267079](https://bugs.openjdk.java.net/browse/JDK-8267079) for detailed description.

-Man

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Commit messages:
 - Improve async handshake

Changes: https://git.openjdk.java.net/jdk/pull/4005/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4005&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267079
  Stats: 32 lines in 3 files changed: 13 ins; 0 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4005.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4005/head:pull/4005

PR: https://git.openjdk.java.net/jdk/pull/4005