datastore: Add DecodeCursor to [Raw]Interface.

service/datastore/raw_interface.go

Index: service/datastore/raw_interface.go
diff --git a/service/datastore/raw_interface.go b/service/datastore/raw_interface.go
index e9c77c6924433fe29bfaa21dbeaa90a31a62ffbf..09eca7ee3e297b344df94a85e94d178689f93bb0 100644
--- a/service/datastore/raw_interface.go
+++ b/service/datastore/raw_interface.go
@@ -165,6 +165,11 @@ type RawInterface interface {
 	//   - f is not nil
 	RunInTransaction(f func(c context.Context) error, opts *TransactionOptions) error
 
+	// DecodeCursor converts a string returned by a Cursor into a Cursor instance.
+	// It will return an error if the supplied string is not valid, or could not
+	// be decoded by the implementation.
+	DecodeCursor(s string) (Cursor, error)
+
 	// Run executes the given query, and calls `cb` for each successfully item.
 	//
 	// NOTE: Implementations and filters are guaranteed that: