GH-3955: Mqttv5PahoMessageDrivenChannelAdapter only unsubscribe when cleanStart

[mths1]

GH-3955: Mqttv5PahoMessageDrivenChannelAdapter only unsubscribe when cleanStart

Fixes #3955

• add check to doStop that avoids unsubscribe when cleanStart is false.
• add testcase

[artembilan]

Also see Checkstyle errors:

Error: eckstyle] [ERROR] /home/runner/work/spring-integration/spring-integration/spring-integration-mqtt/src/test/java/org/springframework/integration/mqtt/Mqttv5AdapterTests.java:27:1: 'org.springframework.beans.factory.BeanFactory' should be separated from previous imports. [ImportOrder]
Error: eckstyle] [ERROR] /home/runner/work/spring-integration/spring-integration/spring-integration-mqtt/src/test/java/org/springframework/integration/mqtt/Mqttv5AdapterTests.java:52:71: ',' is not followed by whitespace. [WhitespaceAfter]
Error: eckstyle] [ERROR] /home/runner/work/spring-integration/spring-integration/spring-integration-mqtt/src/test/java/org/springframework/integration/mqtt/Mqttv5AdapterTests.java:58:71: ',' is not followed by whitespace. [WhitespaceAfter]
Error: eckstyle] [ERROR] /home/runner/work/spring-integration/spring-integration/spring-integration-mqtt/src/test/java/org/springframework/integration/mqtt/Mqttv5AdapterTests.java:74:69: ',' is not followed by whitespace. [WhitespaceAfter]

You can verify your change locally using Gradle command: gradlew clean :spring-integration-mqtt:check

[artembilan]

... and back-ported to 5.5.x as 36e4fe1 after fixing conflicts.

@mths1 ,

thank you for contribution; looking forward for more!