Introduce DataCollection and DataStream (#125)

This commit does the following:

- Renames DataStream to DataCollection since that describes better the interface
- Introduces DataStream as a streamable item within this collection

src/internal/connector/exchange_data_collection.go

@@ -1,12 +1,36 @@
 package connector
 
-// A DataStream provides an iterator to consume a
-// collection of graph data of the same type (e.g. mail)
-type DataStream interface{}
+import "io"
+
+// A DataCollection represents a collection of data of the
+// same type (e.g. mail)
+type DataCollection interface {
+	// Returns either the next item in the collection or an error if one occurred.
+	// If not more items are available in the collection, returns (nil, nil).
+	NextItem() (DataStream, error)
+}
+
+// DataStream represents a single item within a DataCollection
+// that can be consumed as a stream
+type DataStream interface {
+	// ToReader returns a reader for the data stream
+	ToReader() io.Reader
+	// Provides a unique identifier for this data
+	UUID() string
+}
 
 // ExchangeDataCollection represents exchange mailbox
-// data for a single user. It implements the DataStream
-// interface which allows reading data in the collection
+// data for a single user.
+//
+// It implements the DataCollection interface
 type ExchangeDataCollection struct {
 	user string
 }
+
+// NextItem returns either the next item in the collection or an error if one occurred.
+// If not more items are available in the collection, returns (nil, nil).
+func (*ExchangeDataCollection) NextItem() (DataStream, error) {
+	// TODO: Return the next "to be read" item in the collection as a
+	// DataStream
+	return nil, nil
+}

src/internal/connector/graph_connector.go

@@ -120,8 +120,8 @@ func (gc *GraphConnector) GetUsersIds() []string {
 	return values
 }
 
-// ExchangeDataStream returns a DataStream that the caller can
-// use to stream mailbox data out for the specified user
-func (gc *GraphConnector) ExchangeDataStream(user string) DataStream {
+// ExchangeDataStream returns a DataCollection that the caller can
+// use to read mailbox data out for the specified user
+func (gc *GraphConnector) ExchangeDataCollection(user string) DataCollection {
 	return &ExchangeDataCollection{user: user}
 }