New param in `Task.run(), Task.run_asyn()`: `session_id` which is used as a key to look up various settings in Redis cache.
Currently only used to look up "kill status" - this allows killing a running task, either by `task.kill()`
or by the classmethod `Task.kill_session(session_id)`
For example usage, see the `test_task_kill` in [`tests/main/test_task.py`](https://github.com/langroid/langroid/blob/main/tests/main/test_task.py)