Store cluster splitting probabilities in map rather than updating the cluster.

Originally the cluster splitting probabilities were stored in a reserved word of the corresponding PixelClusters. This however required to update a collection already stored in StoreGate i.e. was not thread-safe.

Now the splitting probabilities are stored in separate maps, which are copied and updated when passed from algorithm to algorithm. The algorithms record the map at the very beginning to allow their tools (PixelClusterOnTrackTool) to access the map via read handles rather than passing the map down the call chain explicitly.

This addresses ATLASRECTS-5294 mostly.