wait for services shutdown

```
During emulation shutdown, the shutdown command for each service is called, but
sometimes these commands do not have a chance to complete.

> I noticed that when I shutdown an experiment the nodes are killed
> before each shutdown command returned, and thus the shutdown command
> gets killed itself. Is there a way to wait for all shutdown commands to
> finish before the node is killed?
>
> I have a database running on the nodes, which needs a few seconds to
> shutdown completely.

```

Original issue reported on code.google.com by `ahrenh...@gmail.com` on 5 Apr 2011 at 2:12