Unifying the channel and transport layers to use different data types

For Add, Modify and Delete operations, we have different data types of InstanceInfo, Delta and InstanceIdentifier (wrapper around the String id). Unifying these so that the transport layer can handle them.
Item sounds too abstract. Do you want to look at the existing `ModifyNotification` ?

The positive is that we do not have to get into the generic type to understand what is the type of the data.