Merge r1597533, r1649491, r1665216, r1756553, r1756631, r1726675, r1718496,...

Merge r1597533, r1649491, r1665216, r1756553, r1756631, r1726675, r1718496, r1718476, r1747469 from trunk:

mod_cache: try to use the key of a possible open but stale cache entry
if we have one in cache_try_lock(). PR 50317

Submitted by: Ruediger Pluem


* modules/cache/mod_socache_memcache.c (socache_mc_store): Pass
  through expiration time.

Submitted by: Faidon Liambotis <paravoid debian.org>, jorton


* mod_cache: Preserve the Content-Type in case of 304 response.
304 does not contain Content-Type and mod_mime regenerates
the Content-Type based on the r->filename. This later leads to original
Content-Type to be lost (overwriten by whatever mod_mime generates).


mod_cache: Use the actual URI path and query-string for identifying the
cached entity (key), such that rewrites are taken into account when
running afterwards (CacheQuickHandler off).  PR 21935.
 


mod_cache: follow up to r1756553: log the real/actual cached URI (debug).



better s-maxage support
 
+  *) mod_cache: Consider Cache-Control: s...