TrigSteeringEventTPCnv+xAODTriggerAthenaPool: Implementing MT thinning.

Previously, HLTResult objects were thinned either using the slimming functionality of ThinningSvc or by modifying it in-place using an algorithm. Neither of these will work for MT jobs, and ThinningSvc is anyway being removed.

Add a hook to the TP converter to query ThinningCache for TrigNavigation thinning and apply it if we find it.

Do a similar thing for the xAOD case.