[FLINK-5890] [gelly] GatherSumApply broken when object reuse enabled

The initial fix for this ticket is not working on larger data sets.

Reduce supports returning the left input, right input, a new object, or
a locally reused object. The trouble with the initial fix was that the
returned local object was reusing fields from the input tuples.

The problem is with ReduceDriver#run managing two values (reuse1 and
reuse2) and with a third, local value returned by
GatherSumApplyIteration.SumUDF. After the first grouping value.f1 ==
reuse1.f1. Following UDF calls may swap value.f1 and reuse2.f1, which
causes reuse1.f1 == reuse2.f1. With an odd number of swaps the next
grouping will reduce with reuse1 and reuse2 sharing a field and
deserialization will overwrite stored values.

The simple fix is to only use and return the provided inputs.

This closes #3515

flink-libraries/flink-gelly/src/main/java/org/apache/flink/graph/gsa/GatherSumApplyIteration.java

@@ -330,7 +330,6 @@ private SumUdf(SumFunction<VV, EV, M> sumFunction, TypeInformation<Tuple2<K, M>>
 
 		@Override
 		public Tuple2<K, M> reduce(Tuple2<K, M> arg0, Tuple2<K, M> arg1) throws Exception {
-			K key = arg0.f0;
 			M result = this.sumFunction.sum(arg0.f1, arg1.f1);
 
 			// if the user returns value from the right argument then swap as
@@ -339,9 +338,11 @@ public Tuple2<K, M> reduce(Tuple2<K, M> arg0, Tuple2<K, M> arg1) throws Exceptio
 				M tmp = arg1.f1;
 				arg1.f1 = arg0.f1;
 				arg0.f1 = tmp;
+			} else {
+				arg0.f1 = result;
 			}
 
-			return new Tuple2<>(key, result);
+			return arg0;
 		}
 
 		@Override