gstlal_feature_extractor: changed use_kafka option to be part of save_format...

gstlal_feature_extractor: changed use_kafka option to be part of save_format instead, to allow only one data transfer/saving option to run at a given time, fixed issue with hdf5 file saving where there were repeated timestamps, cleaned up finalization of saving features to disk when pipeline finishes, changed some default save/persist cadences