kiji-schema: SCHEMA-661. Add Closeable to KijiTableReaderPool

issue: https://jira.kiji.org/browse/SCHEMA-661
review: https://review.kiji.org/r/1657
commit: https://github.com/kijiproject/kiji-schema/commit/84ccbbe35ac2d2e15737f57733d5678354af6a55