Set event handler after receiving acknowledgement from Grandeur.

[instanceofMA]

Right now, we call subscriptions.on() right as we send subscription request message to Grandeur. The request could get rejected. Therefore, we should call it after we receive acknowledgement from Grandeur. For that, we can add a task which would schedule a function calling subscriptions.on(). This can be done using a lambda function.

But our Callback class doesn't accept a lambda functions as of now. So we need to redesign the Callback to accept lambda functions.