kiji-rest: REST-121. Ensure that no methods may be called on KijiManagedClient after stop() and that all instances are cleaned up before stop() returns

issue: https://jira.kiji.org/browse/REST-121
review: https://review.kiji.org/r/1790
commit: https://github.com/kijiproject/kiji-rest/commit/72f402ca902a3f63bfc5c12b7503b0e1240692a2