RFR: 8293713 : java/net/httpclient/BufferingSubscriberTest.java fails in timeout, blocked in submission publisher

[Viktor Klang]

Resolves a visibility issue in SubmissionPublisher by making the `waiter` member of BufferedSubscription *volatile*.

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Commit messages:
 - Addresses JDK-8293713 by making the waiter thread member volatile

Changes: https://git.openjdk.org/jdk/pull/16313/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16313&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293713
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16313.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16313/head:pull/16313

PR: https://git.openjdk.org/jdk/pull/16313