Make FileSystemTileCache persistent and expire locally rendered tiles

Signed-off-by: mvglasow <michael -at- vonglasow.com>
FileCache: Do not unconditionally return null if no file is in memory

Signed-off-by: mvglasow <michael -at- vonglasow.com>
FileCache: TTL for downloaded tiles

Signed-off-by: mvglasow <michael -at- vonglasow.com>