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);
  }
 }