Link graph: Explicitly flag invalidated flow stats instead of minimising their flows

Entirely exclude invalidated flow stats from link stats

Delete invalidated flow stats if they stay invalid for 32 link graph jobs

This is to prevent large numbers of invalidated flow stats from
unduly influencing link statistics

src/saveload/extended_ver_sl.h

@@ -77,6 +77,7 @@ enum SlXvFeatureIndex {
 	XSLFI_TOWN_CARGO_MATRIX,                      ///< Town cargo matrix savegame format changes
 	XSLFI_STATE_CHECKSUM,                         ///< State checksum
 	XSLFI_DEBUG,                                  ///< Debugging info
+	XSLFI_FLOW_STAT_FLAGS,                        ///< FlowStat flags
 
 	XSLFI_RIFF_HEADER_60_BIT,                     ///< Size field in RIFF chunk header is 60 bit
 	XSLFI_HEIGHT_8_BIT,                           ///< Map tile height is 8 bit instead of 4 bit, but savegame version may be before this became true in trunk