Improve timestamp handling for rendered tiles

FileSystemTileCache: leave file timestamp alone
DatabaseRenderer, TileRendererLayer: use filesystem timestamp for map database, not the one stored in data header
Some comment cleanup

Signed-off-by: mvglasow <michael -at- vonglasow.com>
Set default TTL for TileDownloadLayers

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