Server cannot be interrupted while plan is running

While a plan is running, trying to interrupt the server does nothing until the plan is finished.

```bash
$ blueapi serve
```
\+
```bash
$ blueapi controller run sleep '{"time": 60}'
```
Sending `Ctrl-C`/`sigint` to the server process will display the `INFO: Shutting down` message but then waits the full 60s for the plan to complete before shutting down.