response_cache plugin: return cached response from store unless stale

response age wasn't being taken into account, and cache invalidation request was always being sent; fresh response will stay in the store until expired; when it expires, cache invalidation will be tried (if possible); if invalidated, the new response is put in the store; Bif validated, the body of the cached response is copied, and the cached response stays in the store.

Closes #341 (closed)