fix(waiterHeap): Zero-out popped slice slots

Avoids memory leaks because the waiter holds a function reference.

internal/ztest/clock.go

@@ -193,6 +193,7 @@ func (h *waiterHeap) Pop() interface{} {
 	old := *h
 	n := len(old)
 	x := old[n-1]
+	old[n-1] = waiter{} // avoid memory leak
 	*h = old[:n-1]
 	return x
 }