kiji-scoring: SCORE-124. Make StatisticsGatheringThread empty the queue on shutdown.

issue: https://jira.kiji.org/browse/SCORE-124
review: https://review.kiji.org/r/1226
commit: https://github.com/kijiproject/kiji-scoring/commit/ed68371915e6bf2ef47c98722e38f77a8a81af0a