async def ack(self, tube, task_id):
cmd = tube.cmd("<STR_LIT>")<EOL>args = (task_id,)<EOL>res = await self.tnt.call(cmd, args)<EOL>return res<EOL>
Report task successful execution.

Ack is accepted only from the consumer, which took the task
for execution. If a consumer disconnects, all tasks taken
by this consumer are put back to READY state (released).

Returns tarantool tuple object.