MQTTv5 the client should send DISCONNECT before closing the connecton

Although the paho java client allows the client application to send a DISCONNECT before closing the network connection, paho itself does not do so before closing the connection. The whole point of the reason codes is to allow the peer to record the reason for a connection close.

In almost all cases where a connection has been established, the client should send a DISCONNECT in any case before doing a connection close.

- [ ] Bug exists Release Version 1.2.0 ( Master Branch)
- [ ] Bug exists in MQTTv3 Client on Snapshot Version 1.2.1-SNAPSHOT (Develop Branch)
- [x] Bug exists in MQTTv5 Client on Snapshot Version 1.2.1-SNAPSHOT (Develop Branch)

If this is a bug regarding the Android Service, please raise the bug here instead: https://github.com/eclipse/paho.mqtt.android/issues/new