DNS cache should properly limit TTL #81

This component and its cache support works reasonably well currently.

However, this implementation involves a lot of code because it was actually written before the underlying cache component supports TTL values. Once the underlying cache supports proper TTL values for its cache values via reactphp/cache#11, we should update this component to take advantage of this.

In particular, we should carefully follow the spec to apply proper TTL values, such as when a single query returns two records with varying TTLs.

Opening this ticket as a reminder – the implementation actually depends on reactphp/cache#11