api: task_manager: add /task_manager/drain

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.

docs/dev/task_manager.md

@@ -76,6 +76,8 @@ Briefly:
         gets or sets new ttl.
 - `/task_manager/user_ttl` -
         gets or sets new user ttl.
+- `/task_manager/drain/{module}` -
+        unregisters all finished local tasks in the module.
 
 # Virtual tasks
 

docs/operating-scylla/admin-tools/task-manager.rst

@@ -89,6 +89,8 @@ API calls
 
 	- *user_ttl* - new user ttl value.
 
+* ``/task_manager/drain/{module}`` - unregisters all finished local tasks in the module.
+
 Cluster tasks are not unregistered from task manager with API calls.
 
 Tasks API

docs/operating-scylla/nodetool-commands/tasks/drain.rst

@@ -0,0 +1,21 @@
+Nodetool tasks drain
+====================
+**tasks drain** - Unregisters all finished local tasks from the module.
+If a module is not specified, finished tasks in all modules are unregistered.
+
+Syntax
+-------
+.. code-block:: console
+
+   nodetool tasks drain [--module <module>]
+
+Options
+-------
+
+* ``--module`` - if set, only the specified module is drained.
+
+For example:
+
+.. code-block:: shell
+
+   > nodetool tasks drain --module repair

docs/operating-scylla/nodetool-commands/tasks/index.rst

@@ -5,6 +5,7 @@ Nodetool tasks
    :hidden:
 
    abort <abort>
+   drain <drain>
    user-ttl <user-ttl>
    list <list>
    modules <modules>
@@ -32,6 +33,7 @@ Supported tasks suboperations
 -----------------------------
 
 * :doc:`abort </operating-scylla/nodetool-commands/tasks/abort>` - Aborts the task.
+* :doc:`drain </operating-scylla/nodetool-commands/tasks/drain>` - Unregisters all finished local tasks.
 * :doc:`user-ttl </operating-scylla/nodetool-commands/tasks/user-ttl>` - Gets or sets user_task_ttl value.
 * :doc:`list </operating-scylla/nodetool-commands/tasks/list>` - Lists tasks in the module.
 * :doc:`modules </operating-scylla/nodetool-commands/tasks/modules>` - Lists supported modules.