v3: MQTTNewConnection.withConnection

[adam-fowler]

The init and connect functions of MQTTNewConnection should be considered internal. The public API should be a withConnection style function which is supplied a closure to run using the generated connection. When you exit the closure a DISCONNECT is sent and the connection is closed eg

MQTTNewConnection.withConnection { connection in
    connection.publish(to: "MyTopic", payload: ByteBuffer(string: "Testing"))
}