[Pubsub] Add cleanup of background threads (GoogleCloudPlatform#1247)

* Shut down daemon threads

Adding `-Dexec.cleanupDaemonThreads=false` in `mvn exec:java` fixes `java.lang.IllegalThreadStateException`.

* Undo changes in commands

* Add cleanupDaemonThreads in pom

pubsub/cloud-client/README.md

@@ -47,7 +47,7 @@ Publishes 5 messages to the topic `my-topic`.
 ```
    mvn exec:java -Dexec.mainClass=com.example.pubsub.SubscriberExample -Dexec.args=my-sub
 ```
-Subscriber will continue to listen on the topic and print out message id and data as messages are received. Press `Ctrl+C` to exit the application. 
+Subscriber will continue to listen on the topic and print out message id and data as messages are received. Press `Ctrl+C` to exit the application.
 
 #### Testing
 Run the test with Maven.

pubsub/cloud-client/pom.xml

@@ -56,5 +56,30 @@
       <version>0.42</version>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>org.apache.maven</groupId>
+      <artifactId>maven-plugin-api</artifactId>
+      <version>3.3.3</version>
+    </dependency>
   </dependencies>
+
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.codehaus.mojo</groupId>
+        <artifactId>exec-maven-plugin</artifactId>
+        <version>1.6.0</version>
+        <executions>
+          <execution>
+            <goals>
+              <goal>java</goal>
+            </goals>
+          </execution>
+        </executions>
+        <configuration>
+          <cleanupDaemonThreads>false</cleanupDaemonThreads>
+        </configuration>
+      </plugin>
+    </plugins>
+  </build>
 </project>