Draft: Ensure traffic weighting still applies with session affinity.

[tmcg-gusto]

This is a first pass at a change to how the Session Affinity cookies work alongside canary weighting, only implemented for gateway api at this point.

As mentioned in this issue, with session affinity enabled, over time all traffic will be sent to the canary, even with the canary weight only set to 10%. Every request a user sends in this scenario will result in a 10% chance they're routed to the canary, after which they'll be on that version until the canary completes, meaning canary will be receiving 100% after a short time (depending on how many requests each user makes).

The approach I've taken here to resolve this is to add another cookie that will hold sessions to the primary until the next step of the analysis, after which a new "primary cookie" will be set, allowing a rebalance according to the next canary weight. This allows the canary weight to stay roughly accurate across the lifetime of the analysis, while still ensuring users don't switch back to primary after hitting the canary version.

I have not handled clearing the primary cookie, which would require storing each cookie defined (or some sort of prefix based match system). Thus far that seems to work fine as the cookie gets overwritten by a new one in the next canary run, but if there are concerns with leaving a cookie set I can make adjustments to the clearing logic.

There's definitely some refactoring to do to make the code more concise, but I wanted to get feedback on the approach prior to investing too much time there.