Cookies: combining multiple prefixes

[yoavweiss]

Following up on a discussion from the mailing list.

As we're planning to add more prefixes, it'd be good to have a way to combine them. After multiple proposals, I think the best way to do that is to extend the prefix syntax to have a single char delimiter (e.g. _), and extend the processing logic to split on that delimiter and process multiple prefixes.

An example cookie name may look like __Host_Http_FlyingCars-cookiename, which will imply that the cookie is host-scoped, http-only and also some future semantics we haven't defined yet (and which will be ignored until we would).

/cc @roryhewitt @annevk @johannhof @cfredric @DCtheTall