This routine just cleans up orphans. We iterate through the filecache, and all files we keep we update the time stamp on (l. 1101). We then call this routine; all files older than the starttime (l. 1055) are not represented by an entry in the cache, are orphans, and can be unlinked. I suppose it is possible for a cache file to be created just before starttime, and put into the cache just after we get our iterator view.
When we PUT a file, or access it and get a 304 or 200, we update last_server_update. If this happens in the past 8 days, ldb_filecache_cleanup will note it, keep, and put a new timestamp on the file, so I think we're ok.