DocumentChange represents a change to the documents matching a query.
It contains the document affected and the type of change that occurred.
The document affected by this change.
The index of the changed document in the result set immediately after
DocumentChange (i.e. supposing that all prior
objects and the current
DocumentChange object have been applied).
Is -1 for 'removed' events.
The index of the changed document in the result set immediately prior to