kiji-schema: SCHEMA-809. Migrate KijiResult from Iterator to KijiResultIterator

issue: https://jira.kiji.org/browse/SCHEMA-809
review: https://review.kiji.org/r/1907
commit: https://github.com/kijiproject/kiji-schema/commit/58aeea6d4a6cc2c0e3e1700ea8f1831d4fde2cf9