RFR: 8357639: DigestEchoClient fails intermittently due to: java.io.IOException: Data received while in pool

[Daniel Fuchs]

The java/net/httpclient/DigestEchoClient.java fails intemittently with IOException: HTTP/1.1 header parser received no bytes.

Analysis shows that this is caused by the CleanupTrigger which receives data after the reused connection has been taken out of the HTTP/1.1 clear pool (Caused by: java.io.IOException: Data received while in pool). This should not happen.

The fix for [JDK-8338569](https://bugs.openjdk.org/browse/JDK-8338569) has improved the situation but apparently didn't fix the issue completely.
The issue is that the write publisher manages to come active before the read subscriber has been switched, which opens a window in which the old read subscriber receives the data that should have been passed to the new one.

To fix that, this change  proposes to pause reading at the selector level before connecting the flows with the new publisher /subscriber pair. This should ensure that data doesn't reach the "old" subscriber, if the new write publisher manage to send data before the new read subscriber is subscribed.

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Commit messages:
 - 8357639

Changes: https://git.openjdk.org/jdk/pull/25416/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25416&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357639
  Stats: 15 lines in 1 file changed: 15 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25416.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25416/head:pull/25416

PR: https://git.openjdk.org/jdk/pull/25416