returned capi_frontend:task_status_all/0

Otherwise common capi debugging endpoint (/_active_tasks) doesn't
work.

Change-Id: Ifc38e48cd9eea397167f854384b044fb76ff30a4
Reviewed-on: http://review.couchbase.org/42431
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>