async
Create a compute that can set its value after the computed function has been called.
    compute.async(initialValue, computed(currentValue, setValue(newValue) )
  
  
  
  Parameters
- The 
{*}:initial value of the compute.
 - computed 
{asyncComputer(lastSetValue, setVal)}:A function that returns the current value of the compute and can optionally later call its
setValuecallback to update the value. 
Returns
 {compute(newVal)}: 
Returns a compute, but a compute that will possibly not have the correct value unless it is bound to.
Use
The following compute is a live list of todos for a given
userId. todos value would alternate between null and a Todo.List as userId changes.
var userId = compute(5)
var todos = compute.async(null, function(oldTodoList, setValue){
  Todo.findAll({ userId: userId() }, function(todos){
    setValue(todos)
  });
  return null;
});
The following replaces the list in place:
var userId = compute(5)
var todos = compute.async(new Todo.List(), function(todoList, setValue){
  todoList.replace( Todo.findAll({ userId: userId() })
  return todoList;
});