Fix speed reduction after critical breakdowns.

Previous code set vcache.cached_max_speed directly (and incorrectly),
which did not survive across save/load or network joins.
Instead add a struct Train field to store the number
of critical breakdowns since last service and do the speed reduction
properly in Train::ConsistChanged.
Slightly tweak algorithm for speed reduction.

src/train.h

@@ -106,6 +106,7 @@ struct Train FINAL : public GroundVehicle<Train, VEH_TRAIN> {
 	TrackBitsByte track;
 	TrainForceProceedingByte force_proceed;
 	RailTypeByte railtype;
+	byte critical_breakdown_count; ///< Counter for the number of critical breakdowns since last service
 	RailTypes compatible_railtypes;
 
 	/** Ticks waiting in front of a signal, ticks being stuck or a counter for forced proceeding through signals. */