fix(debounce): clear emit timer interval on stop

PR #296

If not do this there is possible bug, when timer is set (not executed) and stream stopped if timer interval is not cleared it results in infinite emitting (not sure why emitting is looped).

 I don't have a clean simple repro example, but it happened in my app, had to debug it. Any way seems logical to clear not executed interval on stop, and not to clear is potential wrong.

src/extra/debounce.ts

@@ -18,7 +18,7 @@ class DebounceOperator<T> implements Operator<T, T> {
   _stop(): void {
     this.ins._remove(this);
     this.out = null as any;
-    this.id = null;
+    this.clearInterval();
   }
 
   clearInterval() {