Optimize device/get_changes updates

- don't use folder/get_content but rely on parent information
 - add many more debugging information and sanity checks
 - ignore trash
 - correctly update local device revision using device_response
 - remove "visited" member and change housekeeping function to check for
   consistency of parents and children (and retrieve fix from remote if
   necessary)
 - print key and parent key in the debugging function
 - check for remote updates on every fs function
 - introduce global defines for key and filename length
 - add parent member, getters and setters to file.h

mfapi/file.h

@@ -32,6 +32,10 @@ int             file_set_key(mffile * file, const char *quickkey);
 
 const char     *file_get_key(mffile * file);
 
+int             file_set_parent(mffile * file, const char *parent_key);
+
+const char     *file_get_parent(mffile * file);
+
 int             file_set_hash(mffile * file, const char *hash);
 
 const char     *file_get_hash(mffile * file);