use $ttl for cookie expiration time

I thought about enhancing this sort of usage:
``` php
$f3->set('COOKIE.foo','bar',60); // 1 minute cookie
```

Currently, the $ttl is used to save this key-value pair in cache. I think this isn't very helpful at all.
Instead, it would be cool to bypass the default `JAR.expire` time with this $ttl parameter when setting COOKIE values. So you don't have to change it, if you like to treat a single cookie a bit different. And this would be even more intuitive along the expected behaviour.