kiji-schema: SCHEMA-586. KijiReaderFactory should support getting a reader pool.

issue: https://jira.kiji.org/browse/SCHEMA-586
review: https://review.kiji.org/r/1243
commit: https://github.com/kijiproject/kiji-schema/commit/7724244180d30b201a86de3add75d185a53d6cd7