WorkQueueProcessor leaves threads running after forceShutdown()

[elrodro83]

Expected behavior

forceShutdown() completely shutdowns its inner executor, leaving no threads from it running

Actual behavior

One of the WorkQueueProcessor's inner executor threads keeps running after forceShutdown(), even after being interrupted.

Steps to reproduce

This test (copied from TopicProcessorTest into WorkQueuProcessorTest) reproduces the behavior:

	@Test
	public void testForceShutdownAfterShutdown() throws InterruptedException {
		WorkQueueProcessor<String> processor = WorkQueueProcessor.<String>builder()
				.name("processor").bufferSize(4).build();
		Publisher<String> publisher = Flux.fromArray(new String[] { "1", "2", "3", "4", "5" });
		publisher.subscribe(processor);
		
		AssertSubscriber<String> subscriber = AssertSubscriber.create(0);
		processor.subscribe(subscriber);
		
		subscriber.request(1);
		
		Thread.sleep(250);
		
		processor.shutdown();
		
		assertFalse(processor.awaitAndShutdown(Duration.ofMillis(400)));
		
		processor.forceShutdown();
		
		// This assertion fails
		assertTrue(processor.awaitAndShutdown(Duration.ofMillis(400)));
	}

A thread dump taken before the failing assertion, shows just one of the threads still running:

"processor-1" #16 daemon prio=5 os_prio=0 tid=0x00007f56e0680000 nid=0x390b runnable [0x00007f5721c66000]
   java.lang.Thread.State: RUNNABLE
        at sun.misc.Unsafe.park(Native Method)
        at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:312)
        at reactor.core.publisher.WorkQueueProcessor$WorkQueueInner.run(WorkQueueProcessor.java:505)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1129)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
        at java.lang.Thread.run(Thread.java:748)

   Locked ownable synchronizers:
        - <0x000000076d598d18> (a java.util.concurrent.ThreadPoolExecutor$Worker)

Reactor Core version

3.2.0.M1

JVM version (e.g. java -version)

java version "1.8.0_161"
Java(TM) SE Runtime Environment (build 1.8.0_161-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.161-b12, mixed mode)