RFR: 8309120: java/net/httpclient/AsyncShutdownNow.java fails intermittently

[Daniel Fuchs]

Please find below a fix for a regression observed in the HttpClient AsyncShutdownNow test, after [JDK-8308310](https://bugs.openjdk.org/browse/JDK-8308310)

The PlainHttpConnection should have been modified to use a ReentrantLock instead of trying to perform potentially blocking actions outside of the synchronized block.

I also took this opportunity to fix the regex in the code that validate the exception message. The IOException thrown in that case has "Stream" with a capital "S".

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Commit messages:
 - 8309120

Changes: https://git.openjdk.org/jdk/pull/14223/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14223&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309120
  Stats: 29 lines in 3 files changed: 16 ins; 3 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/14223.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14223/head:pull/14223

PR: https://git.openjdk.org/jdk/pull/14223