Feature request: Add % max_stale_age config

[mlibbey]

Our proxy.config.http.cache.max_stale_age is currently a number of seconds. The shorter the ttl, the larger the percentage of potential stale time. For instance, the default 7 day max_stale_age would be 700% of a 1 day ttl; and obviously grows the shorter the ttl. Conversely, the default is only 25% of a 1 month ttl and 2% of a 1 year percent. It's much safer to allow an object with 1yr ttl to be served an extra 2% than to allow an 1hr ttl object to be served for so long.

We should really have a percentage setting instead-- for instance setting stale to be 25% would allow a 1hr ttl object to be served stale for an extra 15 min, whereas a 1 month ttl object would be allowed to be served for an extra week.