The manager's Close function is prone to race condition

[CermakM]

Describe the bug
When calling the RunTask with a long running task def after Close, the task gets cancelled.

To Reproduce
Use the samples/slog example and add another manager.Close() call after the existing Close call and immediately after add manager.RunTask(ctx, def). The task is executed and killed after a moment.

Expected behavior
One possible solution is to forbid execution of tasks after the manager has been closed (this is a similar philosophy as for other Go Close methods, for example closing a request's body (io.ReadCloser) also does not allow to read from the body anymore (closed socket). I would imagine a lot of people will expect such behavior from a Close method.

If you don't want this kind of behavior, I would consider renaming the method to Stop instead and resolve the race condition.