Fix bug 443744 - MQTTClient generate invalid client id for Websphere MQ 8.0

The client id is valid for MessageSight and also RSMB.  But for some reason, the generated id is not valid for Websphdere MQ 8.0.
The '-' in the clientID prefix is considered as an invalid character in WebSphere MQ server.
This fix removes this character to make it compatible with the most of MQTT brokers.

Bug: 443744
Signed-off-by: bzhangxmt <zhbinbj@cn.ibm.com>

Author: bzhangxmt

org.eclipse.paho.client.mqttv3/src/main/java/org/eclipse/paho/client/mqttv3/MqttAsyncClient.java

@@ -78,7 +78,7 @@ public class MqttAsyncClient implements IMqttAsyncClient { // DestinationProvide
 	private static final String CLASS_NAME = MqttAsyncClient.class.getName();
 	private static final Logger log = LoggerFactory.getLogger(LoggerFactory.MQTT_CLIENT_MSG_CAT,CLASS_NAME);
 
-	private static final String CLIENT_ID_PREFIX = "paho-";
+	private static final String CLIENT_ID_PREFIX = "paho";
 	private static final long QUIESCE_TIMEOUT = 30000; // ms
 	private static final long DISCONNECT_TIMEOUT = 10000; // ms
 	private static final char MIN_HIGH_SURROGATE = '\uD800';
@@ -791,7 +791,7 @@ public void setCallback(MqttCallback callback) {
 	}
 
 	/**
-	 * Returns a randomly generated client identifier based on the the fixed prefix (paho-)
+	 * Returns a randomly generated client identifier based on the the fixed prefix (paho)
 	 * and the system time.
 	 * <p>When cleanSession is set to false, an application must ensure it uses the
 	 * same client identifier when it reconnects to the server to resume state and maintain
@@ -800,7 +800,7 @@ public void setCallback(MqttCallback callback) {
 	 * @see MqttConnectOptions#setCleanSession(boolean)
 	 */
 	public static String generateClientId() {
-		//length of nanoTime = 15, so total length = 20  < 65535(defined in spec) 
+		//length of nanoTime = 15, so total length = 19  < 65535(defined in spec) 
 		return CLIENT_ID_PREFIX + System.nanoTime();
 	}