Determine currency of cached WMS tiles based on response headers

[keithamoss]

This issue relates to 'Permit forwarded WMS requests to be cached' https://github.com/mapserver/mapcache/issues/34#issuecomment-6738078.

When mapcache caches the response of a WMS request it should use either max-age within the Cache-Control header (preferable), or the Expires header, to compute a timestamp to store in its cache. When a second request is made for the same tile the stored timestamp should be consulted first to determine if the request needs to be reissued to the service providing the WMS.

Owing to the need to store the expiry timestamp I am guessing this might have to be limited to the caches that use a database (Sqlite, BerkeleyDB, et al).