kiji-mapreduce: KIJIMR-263. KijiTableKeyValueStore.TableKVReader should use a pool of readers for threadsafety

issue: https://jira.kiji.org/browse/KIJIMR-263
review: https://review.kiji.org/r/1658
commit: https://github.com/kijiproject/kiji-mapreduce/commit/179df0cabe85687b6df11bf87ee09ee86563f43c