add mpe_address to GetChannelState authentication

[astroseger]

We should add mpe_address to the message in GetChannelState authentication.

So the client should sign the following message: __get_channel_state , mpe_address, Channel ID, Current block

In state_service.go in GetChannelState function we should compose the message as following:

    message := bytes.Join([][]byte{
        []byte("__get_channel_state"),
        service.serviceMetaData.GetMpeAddress().Bytes(),
        bigIntToBytes(channelID),
        abi.U256(big.NewInt(int64(request.CurrentBlock))),
    }, nil)