Change TaskSpec to allow multiple object IDs per argument. (#1204)

* Implement object ID bags

* linting

* fix tests

* fix linting

* fix comments

src/common/format/common.fbs

@@ -17,8 +17,11 @@ enum ResourceIndex:int {
 }
 
 table Arg {
-  // Object ID for pass-by-reference arguments.
-  object_id: string;
+  // Object ID for pass-by-reference arguments. Normally there is only one
+  // object ID in this list which represents the object that is being passed.
+  // However to support reducers in a MapReduce workload, we also support
+  // passing multiple object IDs for each argument.
+  object_ids: [string];
   // Data for pass-by-value arguments.
   data: string;
 }