query*Features should preserve non-integer feature IDs

Nobody has claimed this yet.

Here's a jsbin demonstrating the issue:
https://jsbin.com/bafewapela/edit?html,console,output

The GeoJSON spec [states](http://geojson.org/geojson-spec.html#feature-objects) that features may have a top-level id property:

> If a feature has a commonly used identifier, that identifier should be included as a member of the feature object with the name "id".

However, when I have a source with such a feature:

``` javascript
var markers = {
"type": "FeatureCollection",
"features": [{
"id": "my-feature-id", // <--- feature has an ID
"type": "Feature",
"properties": {
"marker-symbol": "theatre",
},
"geometry": {
"type": "Point",
"coordinates": [-77.038659, 38.931567]
}
}]
};
```

add it to the map and then get features back via `queryRenderedFeatures`, the `id` is gone. The `properties` and `geometry` properties are there, but `id` is not. It would be really helpful if this special property were preserved.

The workaround is to duplicate IDs into `properties.id` before adding features to the map, but this creates extra work and contradicts the GeoJSON spec.

**mapbox-gl-js version**: 0.19.1

Start by reproducing the missing top-level id with the JSBin example and trace the queryRenderedFeatures entry point. Verify how feature data is assembled, then preserve non-integer feature IDs in the returned features without requiring properties.id; done means the string ID remains available in queryRenderedFeatures results.

Written by the indexing model from the issue text.