Modernize jetty-util-ajax

[sbordet]

jetty-util-ajax should be modernized and use generics in various places where it does not.

[sbordet]

Users of jetty-util-ajax had to suppress warnings in these cases:

• Implementing JSON.Convertor:

public static class MyConvertor implements JSON.Convertor {
    ...
    @Override
    @SuppressWarnings("rawtypes")
    public Object fromJSON(Map map) {
        ...
    }
}

Same was happening when overriding few JSON methods such as Map newMap() (returning a raw Map), etc.

• Historycally, the JSON class was supposed to be just a collection of static methods. But with the introduction of JSON.Convertor it holds a state, so its static usage was confusing because the static instance is accessible, can be passed around, but it's not properly synchronized, and at risk to be configured differently and unexpectedly by different parts of code.
  Therefore static methods are now deprecated and only instance methods are kept.