When storing parsoid output in the parser cache, we need to set an appropriate cache expiry time, taking into account any dynamic content embedded in the input (compare ParserOutput::getCacheExpiry).
This cache expiry time should also be use for the Cache-Control header of the HTTP resonse, together with expiry info coming from RevisionContentHelper to account for supressed (private!) revisions. (make sure we are converting correctly between absolute time stamp and remaining seconds). We should also send the must-revalidate flag.
NOTE: cache expiry is currently hard-coded to 5 seconds in PageContentHelper. Find out why, and if we can change it.
NOTE: Consult with the traffic team! The cache control header currently received from RESTbase (via varnish) is: `s-maxage=1209600, max-age=0, must-revalidate`.
NOTE: We probably want to mark the stashed flavor (etag has /stash at the end) as non-cacheable, to avoid dup,icate cache entries.