Change the interface for setting application settings

The interface `app.set(, )` feels a odd for setting values in a JavaScript framework. Also, its counterpart `app.get()` is a confusing twin of `app.get(path, callback [, callback ...])`, which is a router method.

I am proposing to implement application settings using getter-setter interfaces.

```
app.settings.trustProxy = true;
```

And get rid of `app.get()`.

Super major change, I know, but the next major version is a good time to make these interface improvements.

**Related issues**

1. https://github.com/expressjs/express/issues/2216
2. https://github.com/expressjs/express/issues/3997