Refactoring of StreamingSparseFeatures: Removed current_vector and current_length
* Removed code duplication; simpler initialization.
* Using non-refcounted sparse vector to avoid double-frees when using get_vector().
Merge pull request #1449 from tklein23/refactoring_streaming_sparse_features

Refactoring of StreamingSparseFeatures: Removed members current_vector/_length