Currently, when the status of a task is queried and the task is already finished,
it gets unregistered. Getting the status shouldn't be a one-time operation.
Stop removing the task after its status is queried. Adjust tests not to rely
on this behavior. Add task_manager/drain API and nodetool tasks drain
command to remove finished tasks in the module.
Fixes: https://github.com/scylladb/scylladb/issues/21388.
It's a fix to task_manager API, should be backported to all branches
Closesscylladb/scylladb#22310
* github.com:scylladb/scylladb:
api: task_manager: do not unregister tasks on get_status
api: task_manager: add /task_manager/drain
d8b8a6c5fc