Yield list of hits instead of individual hits

[sethmlarson]

Instead of yielding individual hits from operations._search_yield_hits() we should instead yield list of hits which are then sent through _es_results_to_pandas to transform a list of hits to a single dataframe. We do this mostly because post-processing is more efficient if done on larger dataframes instead of smaller single-hit ones.

Related to #369 this will help performance because we won't be doing post-processing on individual dataframes.

cc @V1NAY8 @kxbin