RFR: 8294916: Cancelling a request must eventually cause its response body subscriber to be unregistered

[Daniel Fuchs]

When [JDK-8277969](https://bugs.openjdk.org/browse/JDK-8277969) was implemented a list of outstanding response subscribers was added to HttpClientImpl. A body subscriber is added to the list after being created and is removed from the list when it is completed, either successfully or exceptionally.

It appears that in the case where the subscription is cancelled before the subscriber is completed, the subscriber might remain registered in the list forever, or at least until the HttpClient gets garbage collected. This can be easily reproduced using streaming subscribers, such as BodySubscriber::ofInputStream. In the case where the input stream is closed without having read all the bytes, Subscription::cancel will be called. Whether the subscriber gets unregistered or not at that point becomes racy.

Indeed, the reactive stream specification doesn't guarantee whether onComplete or onError will be called or not after a subscriber cancels its subscription. Any cleanup that would have been performed by onComplete/onError might therefore need to be performed when the subscription is cancelled too.

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Commit messages:
 - Merge branch 'master' into inputstream-leak-8294916
 - 8294916

Changes: https://git.openjdk.org/jdk/pull/10659/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10659&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294916
  Stats: 552 lines in 8 files changed: 536 ins; 1 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/10659.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10659/head:pull/10659

PR: https://git.openjdk.org/jdk/pull/10659