MINOR: tasks: Change the task API so that the callback takes 3 arguments.

MINOR: tasks: Change the task API so that the callback takes 3 arguments.

In preparation for thread-specific runqueues, change the task API so that
the callback takes 3 arguments, the task itself, the context, and the state,
those were retrieved from the task before. This will allow these elements to
change atomically in the scheduler while the application uses the copied
value, and even to have NULL tasks later.