fix(groupBy): ensures durationSelector subscriptions are cleaned

The subscriptions to the durationSelector would pile up in the
internal subscription list of the GroupBySubscriber. By maintaining
a map entry per group we can discard subscriptions of a single
group.

Fixes ReactiveX#2661

src/operator/groupBy.ts

@@ -241,6 +241,7 @@ class GroupDurationSubscriber<K, T> extends Subscriber<T> {
     }
     this.parent.removeGroup(this.key);
     this.unsubscribe();
+    this.parent.remove(this);
   }
 
   protected _complete(): void {
@@ -250,6 +251,7 @@ class GroupDurationSubscriber<K, T> extends Subscriber<T> {
     }
     this.parent.removeGroup(this.key);
     this.unsubscribe();
+    this.parent.remove(this);
   }
 }