RFR: 8277969: HttpClient SelectorManager shuts down when custom Executor rejects a task [v2]

[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 incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - 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:
  - all: https://git.openjdk.java.net/jdk/pull/7196/files
  - new: https://git.openjdk.java.net/jdk/pull/7196/files/9ad98bdf..6c6f3f1a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7196&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7196&range=00-01

  Stats: 66043 lines in 1879 files changed: 45834 ins; 10687 del; 9522 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