Specify which executor a task request runs on

[WardLT]

Our mapping between "task" and "which resource it runs on" should be more granular. Right now, all tasks of a certain type run on a certain set of executors. There are examples where changing an input to the task change which resources it can use effectively. Allowing the Thinker to specify a resource and the Task Server respect it would make it easier to perform such resource targeting.