pickle may be unsafe for loading data. All we want is a cookie jar, we
can just serialize its cookies, not necessarily the whole jar with its
type and policy.

Old format is base64(compress(pickle(jar))).
New format is base64(compress(json(jar))) where JSON conversion is on
the list of cookies (taking name, value, domain, path, secure (https)
and expires).

dump_state will now save using the new format while load_state supports
both new and old format. This allows compatibility for some time, then
later old format support in load_state will be dropped too.