RFR: 8372198: Avoid closing PlainHttpConnection while holding a lock [v9]

[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 incrementally with one additional commit since the last revision:

  test update: remove left over from debug session

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28430/files
  - new: https://git.openjdk.org/jdk/pull/28430/files/3530d078..04469dee

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28430&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28430&range=07-08

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