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.
(cherry picked from commit e37d1bcb98)
5d16157936