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.cpp

@@ -111,6 +111,7 @@ const SlxiSubChunkInfo _sl_xv_sub_chunk_infos[] = {
 	{ XSLFI_TOWN_CARGO_MATRIX,      XSCF_NULL,                1,   1, "town_cargo_matrix",         nullptr, nullptr, nullptr        },
 	{ XSLFI_STATE_CHECKSUM,         XSCF_NULL,                1,   1, "state_checksum",            nullptr, nullptr, nullptr        },
 	{ XSLFI_DEBUG,                  XSCF_IGNORABLE_ALL,       1,   1, "debug",                     nullptr, nullptr, "DBGL"      },
+	{ XSLFI_FLOW_STAT_FLAGS,        XSCF_NULL,                1,   1, "flow_stat_flags",           nullptr, nullptr, nullptr        },
 	{ XSLFI_NULL, XSCF_NULL, 0, 0, nullptr, nullptr, nullptr, nullptr },// This is the end marker
 };