When rewriting cache.config in YAML, make it not additive.

cache.config is currently additive.

https://docs.trafficserver.apache.org/en/9.0.x/admin-guide/files/cache.config.en.html#matching-multiple-rules

> When multiple rules are specified in cache.config, Traffic Server will check all of them in order for each request. Thus, two rules which match the same request but have conflicting actions will result in their actions being compounded. In other words, Traffic Server does not stop on the first match.

This is highly unintuitive and unexpected. Furthermore, because the match logic isn't very powerful, it's not possible to do certain things.

I'm making this issue as a placeholder so we don't forget: when we rewrite it in YAML, we should make it not additive, or possibly extend the matching capabilities to be more powerful, so things like `suffix=jpeg` and `suffix!=jpeg` are possible, or possibly both.