RFR: 8277969: HttpClient SelectorManager shuts down when custom Executor rejects a task [v5]

[Daniel Fuchs]

> These changes make sure that pending requests are terminated if the selector manager thread exits due to exceptions.
> This includes:
>    1. completing CompletableFutures that were returned to the caller code
>    2. cancelling requests that are in flight
>    3. calling onError on BodySubscribers that may not have been completed
> Note that step 3 is necessary as certain CompletableFutures, such as those returned by `BodySubscribers.ofInputStream`, complete immediately, the operation being eventually completed when the last bite of the response is read. Completing a completable future that is already completed has no effect, this case is handled by completing the BodySubscriber too.

Daniel Fuchs has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains eight commits:

 - Merge branch 'master' into executor-shutdown-8277969
 - Merge branch 'master' into executor-shutdown-8277969
 - Merge branch 'master' into executor-shutdown-8277969
 - Merge
 - asserts that acquired == true
 - Incorporated review comments
 - Merge branch 'master' into executor-shutdown-8277969
 - 8277969: HttpClient SelectorManager shuts down when custom Executor rejects a task

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Changes: https://git.openjdk.java.net/jdk/pull/7196/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7196&range=04
  Stats: 1975 lines in 23 files changed: 1715 ins; 139 del; 121 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7196.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7196/head:pull/7196

PR: https://git.openjdk.java.net/jdk/pull/7196