Rescan CDB cache folder after saving cache on disk

Created by: Xottab-DUTY

After testing CDB cache (that was added in #698), we (me and testers) had found a small but sensible bug: If we go, e.g., from the Zaton to the Yanov and back to the Zaton, the game won't find cached files until game restart.

This happens because FS.exist(*cdb_cache_file*) always return false in the current game session even if the file exist, because the filesystem doesn't know that they exist. Of course, after game restart (full application restart), the filesystem scans all the files and finds CDB cache and starts to work correctly.

Filesystem supports folders rescanning. Just need to use it.