kiji-express: EXP-316. memory leak when reading from cells with many versions

issue: https://jira.kiji.org/browse/EXP-316
review: https://review.kiji.org/r/1411
commit: https://github.com/kijiproject/kiji-express/commit/d979fd147d14969b9b081034eecde27b3ff9aab9