[WF4] Simplification of initialization of variables and arguments

-New solution is much cleaner utilising Schedule Activity with callback
feature of runtime
-Cleans up :
  -Task handling: no longer a concept of an initialization task
  -Argument handling: no longer special handling for "Result" arg of
   activities passed in initialization tasks to WorkflowRuntime
  -l value handling: no longer temporary storage of references retrieved
   and the need to call HandleReferences before execution of activities
  -Setting of "Result" argument for CodeActivity<T> no longer requires
   dedicated method in ActivityContext so it can write default values
   to readonly variables