Cache expiry seconds

A shared CDN edge has just stored a response and must decide how many seconds it may serve that copy before asking the origin again. The response's cache directives arrive as separate strings, each either a bare name such as public or a name and value joined by an equals sign such as max-age=600. A directive may carry surrounding spaces and any letter case. Apply these rules in this order and return the number of seconds. Return -1 when no-store or private appears anywhere, since a shared cache may not keep the response at all. Return 0 when no-cache appears, since every hit must be revalidated. Otherwise return the value of s-maxage, then the value of max-age, using the first occurrence of a repeated name and skipping any value that is not a run of digits. Otherwise return default_ttl. Ignore all other directives.