Add `query()` method to `server::Request`

[seanmonstar]

Similar to #896

fn query(&self) -> Option<&str>;

[hagsteel]

What about using a hashmap for the query string?
I imagine that any time you want to work with the querystring you want to work with the values.

e.g pagination

let page = request.query["page"].unwrap();

[seanmonstar]

You might want the query params, but not necessarily. A somewhat common usecase for a query string is as a cache buster, i.e. /foo/bar.js?abcdef, such as adding a timestamp or sha to the end. I'd also prefer leaving the choice of what to parse them into up to the user. Whether that is Vec<(Name, Value)>, a HashMap, or whatever.