Add a "score_mode" parameter to rescoring query.

Default value is "total", possible values are: "max", "min", "avg",
"multiply" and "total".

- "total": the final score of a document is the sum of the original
  query score with the rescore query score.
- "max": only the highest score count.
- "min": only the lowest score is kept (if the document doesn't match
  the rescore query, the original query score is used).
- "avg": average of both scores
- "multiply": product of both scores

Closes #3258