RFR: 8372198: Avoid closing PlainHttpConnection while holding a lock [v8]

[Daniel Fuchs]

> An experimental change to SelectorManager::shutdown unveiled a potential deadlock between the SelectorManager thread trying to stop the HttpClientImpl, and an executor thread concurrently trying to return a connection to the pool.
> 
> The issue seems to be caused by the ConnectionPool::returnToPool trying to close the returned connection when stopping, while holding the ConnectionPool state lock, and the SelectorManager thread trying to close a pooled connection, holding the connection stateLock and trying to close the channel, which caused the CleanupTrigger to fire and attempt to remove the connection from the pool.
> 
> This problem was observed once with the java/net/httpclient/ThrowingSubscribersAsLimitingAsync.java test.
> 
> To avoid the problem, the proposed fix is to wait until the ConnectionPool::stateLock has been released before actually closing the connection, and to wait until the PlainHttpConnection::stateLock has been released before actually closing the channel. Indeed, there should be no need to close those while holding the lock.
> 
> This PR was recreated due to a bad merge pushed to https://github.com/openjdk/jdk/pull/28421

Daniel Fuchs has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 14 commits:

 - Merge branch 'master' into ConnectionCloseLock-8372198
 - test updates: add diagnosis, configure the backlog to be at least the number of concurrent requests
 - More test updates
 - test updates
 - Review feedback: added a test
 - Merge branch 'master' into ConnectionCloseLock-8372198
 - Merge branch 'master' into ConnectionCloseLock-8372198
 - Merge master
 - 8372198: Avoid closing PlainHttpConnection while holding a lock
 - Merge branch 'master' into SelectorManagerVT-8372159
 - ... and 4 more: https://git.openjdk.org/jdk/compare/c028369d...3530d078

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Changes: https://git.openjdk.org/jdk/pull/28430/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28430&range=07
  Stats: 431 lines in 4 files changed: 406 ins; 9 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/28430.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28430/head:pull/28430

PR: https://git.openjdk.org/jdk/pull/28430