Use MonoProcessor instead of FluxIdentityProcessor

We just need to signal completion when close() is called. MonoProcessor
should suffice and we can avoid a hard dependency on Reactor 3.4.

See gh-25085

spring-messaging/src/main/java/org/springframework/messaging/tcp/reactor/ReactorNettyTcpClient.java

@@ -32,10 +32,8 @@ import io.netty.util.concurrent.ImmediateEventExecutor;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 import org.reactivestreams.Publisher;
-import reactor.core.publisher.FluxIdentityProcessor;
 import reactor.core.publisher.Mono;
 import reactor.core.publisher.MonoProcessor;
-import reactor.core.publisher.Processors;
 import reactor.core.scheduler.Scheduler;
 import reactor.core.scheduler.Schedulers;
 import reactor.netty.Connection;
@@ -317,7 +315,7 @@ public class ReactorNettyTcpClient<P> implements TcpOperations<P> {
 					logger.debug("Connected to " + conn.address());
 				}
 			});
-			FluxIdentityProcessor<Void> completion = Processors.more().multicastNoBackpressure();
+			MonoProcessor<Void> completion = MonoProcessor.create();
 			TcpConnection<P> connection = new ReactorNettyTcpConnection<>(inbound, outbound,  codec, completion);
 			scheduler.schedule(() -> this.connectionHandler.afterConnected(connection));
 

spring-messaging/src/main/java/org/springframework/messaging/tcp/reactor/ReactorNettyTcpConnection.java

@@ -17,8 +17,8 @@
 package org.springframework.messaging.tcp.reactor;
 
 import io.netty.buffer.ByteBuf;
-import reactor.core.publisher.FluxIdentityProcessor;
 import reactor.core.publisher.Mono;
+import reactor.core.publisher.MonoProcessor;
 import reactor.netty.NettyInbound;
 import reactor.netty.NettyOutbound;
 
@@ -42,11 +42,11 @@ public class ReactorNettyTcpConnection<P> implements TcpConnection<P> {
 
 	private final ReactorNettyCodec<P> codec;
 
-	private final FluxIdentityProcessor<Void> closeProcessor;
+	private final MonoProcessor<Void> closeProcessor;
 
 
 	public ReactorNettyTcpConnection(NettyInbound inbound, NettyOutbound outbound,
-			ReactorNettyCodec<P> codec, FluxIdentityProcessor<Void> closeProcessor) {
+			ReactorNettyCodec<P> codec, MonoProcessor<Void> closeProcessor) {
 
 		this.inbound = inbound;
 		this.outbound = outbound;