kiji-scoring: SCORE-158. Add serialized client request to ScoringServerScoreFunction

issue: https://jira.kiji.org/browse/SCORE-158
review: https://review.kiji.org/r/1656
commit: https://github.com/kijiproject/kiji-scoring/commit/ef9c64dc44a9ff02d8e8ef950f644a9e0e99ae2f