[WIP] Cookies serailizer improvements

- [x] Since the option actually changes how encrypted cookies are serialized, we should name it properly and include the signed cookie jar as well.
- [ ] Better tests. Right now the test coverage is pretty sparse. Need to better test the existing cookie features and make sure they work with both of the built-in serializers.
- [ ] Fix flash.
- [ ] Find out what else is broken by swapping the default serializers, then fix those.
- [ ] Add `HybridSerializer` (need a better name): subclass `JsonSerializer` to transparently migrate existing marshalled cookies into JSON cookies (we can detect the marshal signiture "\x04\x08" and branch accordingly).
- [ ] Update docs, guides, changelogs to reflect this. Also mention the restrictions on what can be reliably stored in the serialized jars.
- [ ] Upgrade guides on migrating existing cookies.