Review how we might migrate to using cookie-rs Cookies directly

[pfernie]

There are some extra fields/parsing requirements user_agent uses (access timestamps, etc.), that may not be appropriate for upstreaming. Furthermore, cookie-rs strives to be no allocation where possible, which may be at odds with some of the normalization/canonicalization steps taken in user_agent.

However, maybe moving some fields to a more aptly named CookieMetadata or similar struct, containing a cookie::Cookie might make sense.