The PROPFIND does not need to check the filesystem for the file existence,
because the list comes from the directory listing, similar as the other
parts already do it. That was highly visible with large collections (10k+ items),
causing massive delays.
Also avoid repeated stat() calls on the same path.
Based on the ldap groups the user is member of group calender usage is implemented.
The group calenders must be placed in the GROUPS directory based under collection_root_folder.
The name of the group calender directory is the base64 encoded name of the group to avoid trouble with spaces and special characters in name.
If the directory does not exist the group will be ignored.
