kiji-mapreduce: KIJIMR-244. KeyValueStoreReader open method isn't thread safe

issue: https://jira.kiji.org/browse/KIJIMR-244
review: https://review.kiji.org/r/1290
commit: https://github.com/kijiproject/kiji-mapreduce/commit/efe423c9dd70baf55f2fd180b089517bb4e5fece