Implemented check_subtask_status to check the status of each task
separately. Modified tasks management. Now if the group task isn't
finished within one second, we get the id of each subtask and manage
them as AsyncTasks.
Modified check_task_status function to manage the subtask.
Renamed groupt task variables for clarity
Added comments in group task id session management.
Modified cancel task section and cancel_task function. Now we get the
remaining tasks not finished from request session and cancel them all.
Modified cancel tasks behaviour. Now if there are no tasks ready yet, we
get the grouptask id and cancel it. If there are tasks ready, we get the
ids of the tasks remaining and cancel them all like AsyncResults.