MqttClient.isConnected() returns true after call to MqttClient.disconnect()

[mhball]

The following Kotlin snippet prints true.

mqttClient.disconnect {
    if(it.succeeded()) {
        println(mqttClient.isConnected)
}

It seems an intentional disconnect doesn't set the isConnected flag to false.

[vietj]

actually it does but once the TCP connection is closed, I think it should be done before

[vietj]

See #179