worker doesn't shutdown when cluster.scale_up is used

using this code

```
from distributed import Client, LocalCluster, Worker
cluster = LocalCluster(processes=True, n_workers=1)
cluster.scale_up(5)
cluster.close()
```
dask workers doesn't shutdown , they go into closing state but doesn't die
ps -ef | grep -i dask
```
lmn 21126 19797 10 Jan10 pts/9 02:51:42 dask-worker [closing]
lmn 21137 19797 10 Jan10 pts/9 02:58:44 dask-worker [closing]
lmn 21145 19797 10 Jan10 pts/9 02:51:31 dask-worker [closing]
lmn 21151 19797 10 Jan10 pts/9 02:51:27 dask-worker [closing]
lmn 25264 19797 10 Jan10 pts/9 02:51:58 dask-worker [closing]
```