Implement lower level XEH cache
Currently the XEH cache is implemented per CfgVehicle class, but each CfgVehicle class inherits eventhandlers from other classes,
this means that even though we do not duplicate processing for multiple entities sharing the same Cfgvehicle class,
we are still compiling and storing entries from Extended_****_EventHandlers over and over, for different CfgVehicle classes.
This should further reduce compile time, and memory usage.