Complete I/O reactor termination when the selector has been closed by another thread

[AdzerKI]

We hit this with HttpClient 5 talking to a TLS 1.3 server that does not answer close_notify (JDK default, jdk.tls.acknowledgeCloseNotify=false). CloseableHttpAsyncClient#close() waits for the graceful shutdown, gives up after 5 seconds, and MultiCoreIOReactor then closes each SingleCoreIOReactor with CloseMode.IMMEDIATE from the calling thread. That closes the selector while the I/O dispatch thread is still running. Its doTerminate calls closeOpenChannels, selector.keys() throws, and the default LoggingExceptionCallback logs an ERROR on every client shutdown:

java.nio.channels.ClosedSelectorException
	at java.base/sun.nio.ch.SelectorImpl.ensureOpen(Unknown Source)
	at java.base/sun.nio.ch.SelectorImpl.keys(Unknown Source)
	at org.apache.hc.core5.reactor.SingleCoreIOReactor.closeOpenChannels(SingleCoreIOReactor.java:418)
	at org.apache.hc.core5.reactor.SingleCoreIOReactor.doTerminate(SingleCoreIOReactor.java:117)
	at org.apache.hc.core5.reactor.AbstractSingleCoreIOReactor.execute(AbstractSingleCoreIOReactor.java:99)
	at org.apache.hc.core5.reactor.IOReactorWorker.run(IOReactorWorker.java:44)

The exception also skips processClosedSessions, so the handlers of sessions closed by close(CloseMode) never get disconnected.

By then close(CloseMode) has already closed the registered channels, so closeOpenChannels now just returns when the selector is closed and termination goes on. doExecute already ignores ClosedSelectorException for the same reason.

Test: TestSingleCoreIOReactor#terminatesWhenSelectorClosedByAnotherThread holds the dispatch thread in connected, closes the reactor from the test thread, then lets the dispatch thread go. It expects no reported exception and a disconnected call. Fails on master with ClosedSelectorException, passes with the fix; httpcore5 builds with all tests.

The same code is on 5.4.x (the stack trace above is from 5.4.3).

[ok2c]

@AdzerKI Cherry-picked to 5.4.x