Description: ------------ To improve the expensive Garbage Collection procedure of Container_file (opening each cache file and reading the expiry date from the file content), there are two possibilities: 1. put the expiry date into the filename and do only directory reads for GC 2. calculate the expiry date from the ctime of each file. This also needs no opening of the cache files Method 2 is probably "nicer", since it does not change the naming convention of cache files.