[FLINK-3485][core] implement toString for SerializedListAccumulator

The change improves the display of SerializedListAccumulators in the web
interface by displaying the number of elements contained in the
accumulator instead of the hash code of the object.

This closes apache#1717.

flink-core/src/main/java/org/apache/flink/api/common/accumulators/SerializedListAccumulator.java

@@ -42,14 +42,14 @@ public class SerializedListAccumulator<T> implements Accumulator<T, ArrayList<by
 
 	private static final long serialVersionUID = 1L;
 
-	private ArrayList<byte[]> localValue = new ArrayList<byte[]>();
-	
+	private ArrayList<byte[]> localValue = new ArrayList<>();
+
 
 	@Override
 	public void add(T value) {
 		throw new UnsupportedOperationException();
 	}
-	
+
 	public void add(T value, TypeSerializer<T> serializer) throws IOException {
 		try {
 			ByteArrayOutputStream outStream = new ByteArrayOutputStream();
@@ -97,4 +97,9 @@ public static <T> List<T> deserializeList(ArrayList<byte[]> data, TypeSerializer
 		}
 		return result;
 	}
+
+	@Override
+	public String toString() {
+		return "SerializedListAccumulator: " + localValue.size() + " elements";
+	}
 }