kiji-scoring: SCORE-120. InternalFreshKijiTableReader should accept an Executor service as a constructor parameter which would override the default singleton threadpool

issue: https://jira.kiji.org/browse/SCORE-120
review: https://review.kiji.org/r/1294
commit: https://github.com/kijiproject/kiji-scoring/commit/c6fb6d6676afc5d95f8739c0f0a0a3cfb0d47b66