CBQE-0: task._Thread__stop() for all tasks if one failed

incremental_rebalance.. tests

Change-Id: I9b550097e29e5a068c3e86587635490d34a3ec59
Reviewed-on: http://review.couchbase.org/36834
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: buildbot <build@couchbase.com>