RFR: 8286194: ExecutorShutdown test fails intermittently

[Daniel Fuchs]

Hi, please find here a patch that solves a rare intermittent test failure observed in the test `java/net/httpclient/ExecutorShutdown.java`

A race condition coupled with some too eager synchronization was causing a deadlock between an Http2Connection close, a thread trying to shutdown the HttpClient due to a RejectedTaskException, and the SelectorManager thread trying to exit.

The fix comprises several cleanup - in particular:

- `Http2Connection`: no need to try to send a `GOAWAY` frame if the underlying TCP connection is already closed
- `SSLFlowDelegate`/`SubscriberWrapper`: no need to trigger code that will request more data from upstream if the sequential scheduler that is supposed to handle that data once it arrives is already closed
- `Http1Exchange`/`Http1Request`: proper cancellation of subscription if an exception is raised before `onSubscribe()` has been called
- `HttpClientImpl`: avoid calling callbacks from within synchronized blocks when not necessary
- `ReferenceTracker`: better thread dumps in case where the selector is still alive at the end of the test (remove the limit that limited the stack traces to 8 element max by no longer relying on `ThreadInfo::toString`)

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Commit messages:
 - 8286194: ExecutorShutdown test fails intermittently

Changes: https://git.openjdk.java.net/jdk/pull/8562/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8562&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286194
  Stats: 135 lines in 7 files changed: 107 ins; 3 del; 25 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8562.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8562/head:pull/8562

PR: https://git.openjdk.java.net/jdk/pull/8562