RFR: 8310330: HttpClient: debugging interestOps/readyOps could cause exceptions and smaller cleanup

[Daniel Fuchs]

Please find here a change that makes sure that debugging interestOps/readyOps won't cause exceptions to be raised.
The change also contains smaller cleanup: extracting ALPN contants to their own class, removing/adding newlines at end of files, avoiding long lines in some occasions, removing some unused imports, etc...

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Commit messages:
 - whitespaces
 - 8310330

Changes: https://git.openjdk.org/jdk/pull/14546/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14546&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310330
  Stats: 216 lines in 32 files changed: 150 ins; 36 del; 30 mod
  Patch: https://git.openjdk.org/jdk/pull/14546.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14546/head:pull/14546

PR: https://git.openjdk.org/jdk/pull/14546