RFR: 8267140: Support closing the HttpClient by making it auto-closable [v12]

[Daniel Fuchs]

> Please find here an RFE that makes the `java.net.HttpClient` auto-closeable.
> 
> The API has been modeled on `ExecutorService`.
> 
> HttpClient::close() is a graceful shutdown and will wait until all operations are terminated before returning.
> If a request is in progress, and the caller doesn't pull the corresponding data (for instance, the request was sent with a BodyHandler.ofInputStream(), but the caller stopped reading the input stream) then close() may never return.
> 
> Therefore, additional methods similar to those present in `ExecutorService` are also proposed. In summary:
> 
> - `shutdown()`: initiate a graceful shutdown, but doesn't wait for termination. 
> - `shutdownNow()`:  initiate an immediate shutdown, attempting to cancel all operations in progress. Doesn't wait for termination.
> - `awaitTermination(Duration)`: await for termination within the given delay
> - `isTerminated()` tells whether the client is terminated
> 
> New tests have been added to test the proposed behaviors.
> 
> HttpClient tests (new and old) are still stable.

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

  Integrated feedback on tests

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/13019/files
  - new: https://git.openjdk.org/jdk/pull/13019/files/2fca2991..6226acff

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13019&range=11
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13019&range=10-11

  Stats: 121 lines in 5 files changed: 43 ins; 4 del; 74 mod
  Patch: https://git.openjdk.org/jdk/pull/13019.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13019/head:pull/13019

PR: https://git.openjdk.org/jdk/pull/13019