Add support for SameSite cookie directive

[vpavic]

This is a WIP branch for #1005.

There are still some failing tests, due to pain of dealing with cookie directives that don't have support in Servlet spec (lik SameSite directive is), but please take a look on the general approach @rwinch.

[vpavic]

Note to self: Since we write Set-Cookie header manually, org.apache.tomcat.util.http.Rfc6265CookieProcessor#generateHeader is probably something worth looking at for reference.