Close listening socket on graceful shutdown

## Feature Request

### Crates

tonic

### Motivation

- listening socket will never be served, so holding it is pointless
- but it is helpful for testing behavior like reconnecting on server restart

### Proposal

In the middle of here:

https://github.com/hyperium/tonic/blob/d4e4a2aaf6de36f5018c5f854d545e0439800b80/tonic/src/transport/server/mod.rs#L768-L770

add

```
drop(incoming);
```

### Alternatives

Use your own incoming and connect it to the same signal.