detect Releasefile IMS hits even if the server doesn't

detect Releasefile IMS hits even if the server doesn't

Not all servers we are talking to support If-Modified-Since and some are
not even sending Last-Modified for us, so in an effort to detect such
hits we run a hashsum check on the 'old' compared to the 'new' file, we
got the hashes for the 'new' already for "free" from the methods anyway
and hence just need to calculated the old ones.

This allows us to detect hits even with unsupported servers, which in
turn means we benefit from all the new hit behavior also here.