CBQE-0: replace task._Thread__stop() -> task.stop()

Change-Id: I48045da0214da90b839d4decf8475646d481af9c
Reviewed-on: http://review.couchbase.org/36838
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: buildbot <build@couchbase.com>