RFR: 8267140: Clarify that HttpClient does not have to be closed / support closing HttpClient

[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.

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Commit messages:
 - 8267140

Changes: https://git.openjdk.org/jdk/pull/13019/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13019&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8267140
  Stats: 1986 lines in 10 files changed: 1962 ins; 2 del; 22 mod
  Patch: https://git.openjdk.org/jdk/pull/13019.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/13019/head:pull/13019

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