kiji-schema: SCHEMA-650. KijiTableReaderPool should retain the table from which its readers read.

issue: https://jira.kiji.org/browse/SCHEMA-650
review: https://review.kiji.org/r/1626
commit: https://github.com/kijiproject/kiji-schema/commit/450400779c6eb8c937a9b0da4deda24c5b2cae0e