Allow customization of key generation in cookie session store

I figured it's better not to rely on the KeyGenerator defaults and have cookie store specific defaults for key generation.

Also, the test 'uses specified key generator opts' is not actually checking the results of `put` against the result the result of MessageEncryptor with keys generated with same params. Thought it would be doing a bit too much. This may be too little though. Let me know and I can put more effort into it.

This PR may create conflicts with the cookie serializers PR. I can resolve the conflicts and resubmit.

Thanks a lot.