RFR: 8277969: HttpClient SelectorManager shuts down when custom Executor rejects a task [v3]

[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 incrementally with one additional commit since the last revision:

  asserts that acquired == true

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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/6c6f3f1a..90ab52de

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7196&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7196&range=01-02

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 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