v3: MQTTChannelHandler.StateMachine

[adam-fowler]

The MQTTChannelHandler should include an internal state machine. Something like this

struct MQTTChannelHandlerStateMachine {
    enum State {
        case uninitialised
        case connected
        case disconnected
    }

    let state: State

    enum ReceivedPacketAction {
        case respond
        case ignore
        case closeConnection
        ...
    }

    func receivedPacket(_ packet: MQTTPacket) -> ReceivedPacketAction {
        switch self.state {
        ...
    }