Currently, /task_manager/task_status_recursive/{task_id} and
/task_manager/task_status/{task_id} unregister queries task if it
has already finished.
The status should not disappear after being queried. Do not unregister
finished task when its status or recursive status is queried.
In the following patches, get_status won't be unregistering finished
tasks. However, tests need a functionality to drop a task, so that
they could manipulate only with the tasks for operations that were
invoked by these tests.
Add /task_manager/drain/{module} to unregister all finished tasks
from the module. Add respective nodetool command.
