Add comments to handler for async handshake

[AdityaSripal]

Writeup in the comments of how to compose the utility functions provided by IBC to implement async handshakes

closes: #732

cc: @michaelfig

[michaelfig]

@AdityaSripal, for async version negotiation in ibc-go, will I have to patch core/keeper/msg_server.go's ChannelOpenTry with the following?

    // Perform application logic callback
    version, err := cbs.OnChanOpenTry(ctx, msg.Channel.Ordering, msg.Channel.ConnectionHops, msg.PortId, channelID, cap, msg.Channel.Counterparty, msg.CounterpartyVersion)
    if err != nil {
        return nil, sdkerrors.Wrap(err, "channel open try callback failed")
    }

    // FIGME: If version is the empty string, expect the app to call WriteOpenTryChannel explicitly, possibly later...
    if version != "" {
        // Write channel into state
        k.ChannelKeeper.WriteOpenTryChannel(ctx, msg.PortId, channelID, msg.Channel.Ordering, msg.Channel.ConnectionHops, msg.Channel.Counterparty, version)
    }

    return &channeltypes.MsgChannelOpenTryResponse{
        Version: version,
    }, nil

This may also justify adding something like a WriteOpenTryChannel method to ICS4Wrapper to extend the async version feature to Middleware.

[AdityaSripal]

@michaelfig yes. We are trying to improve the interface for middleware to avoid the complicated wiring as it exists. I think this will be much cleaner in future designs, and the ICS4 wrapper interface itself should be unnecessary