PileupMT: Pass on overriden event info to output file

Merged Beojan Stanislaus requested to merge bstanisl/athena:master into master

The event info changes made by EvtIdModifierSvc weren't being passed on to the output file, which made it impossible to run RAWtoALL on the produced RDOs.

This MR fixes that. It also reduces the number of high pT minbias events loaded per hard scatter, to be a little more frugal.

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