Core API: easify cookie access (#696).

BREAKING CHANGE

If your plugin needs access to the current request's cookies just call `Cookies.get()`.
This works in any class.
Internally the Core stores the current request (and thus the cookies) in a thread-local variable.

Example:
{{{
import de.deepamehta.core.service.Cookies;

public class YourPlugin ... {

    void someMethod() {
        Cookies cookies = Cookies.get();
        ...
    }
}
}}}

This is much more easy than the recently introduced approach (which worked in root resource classes only):

{{{
import de.deepamehta.core.service.Cookies;

import javax.ws.rs.Path;
import javax.ws.rs.core.Context;
import javax.ws.rs.core.HttpHeaders;

@Path("...")
public class YourPlugin ... {

    @Context
    private HttpHeaders httpHeaders;

    void someMethod() {
        Cookies cookies = new Cookies(httpHeaders);
        ...
    }
}
}}}

See #696.