RFR: 8317808: HTTP/2 stream cancelImpl may leave subscriber registered [v2]

[Daniel Fuchs]

> Some of the ThrowingSubscriberXxx tests (typically those that use a streaming body) have been observed failing intermittently (though rarely) in timeout. The error message printed at the end showed that some HTTP/2 stream subscribers had not been properly deregistered when an exception was thrown in GET_BODY.
> 
> The issue appears to be a race condition between `cancelImpl` and `schedule` where the scheduler `sched` may get stopped before the subscriber is exceptionally completed.
> 
> A simple fix is to ensure that `cancelImpl` will complete the subscriber, if needed.

Daniel Fuchs has updated the pull request incrementally with one additional commit since the last revision:

  Simplify the fix

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16144/files
  - new: https://git.openjdk.org/jdk/pull/16144/files/4cc23df3..a97ebe20

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16144&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16144&range=00-01

  Stats: 9 lines in 1 file changed: 0 ins; 7 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16144.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16144/head:pull/16144

PR: https://git.openjdk.org/jdk/pull/16144