JavaDoc about sessionExpiryInterval in MQTT 5 client is incorrect

- [x] Bug exists Release Version 1.2.5 ( Master Branch)
- [ ] Bug exists in MQTTv3 Client on Snapshot Version 1.2.6-SNAPSHOT (Develop Branch)
- [X] Bug exists in MQTTv5 Client on Snapshot Version 1.2.6-SNAPSHOT (Develop Branch)

The JavaDoc on `org.eclipse.paho.mqttv5.client.MqttConnectionOptions#setSessionExpiryInterval` states (in part):

> By default this value is null and so will not be sent, in this case, the session will not expire.

However [section 3.1.2.11.2 of the MQTT 5 specification](https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901048) states:

> If the Session Expiry Interval is absent the value 0 is used. If it is set to 0, or is absent, the Session ends when the Network Connection is closed.
>
> If the Session Expiry Interval is 0xFFFFFFFF (UINT_MAX), the Session does not expire.

The JavaDoc on `org.eclipse.paho.mqttv5.client.MqttConnectionOptions#setSessionExpiryInterval` should be corrected to reflect the behavior defined in the specification. It would also probably be useful to make `org.eclipse.paho.mqttv5.common.packet.MqttDataTypes#FOUR_BYTE_INT_MAX` public and reference that in the JavaDoc as an easy way to ensure the session does not expire.