Made UpgradeGhostId thread safe

Note that the interface it implements is not thread safe due to the exitence of beginEvent. It's usage by TrackBestTrackCreator is also not thread safe, with an explicit const_cast introduced in 2016 to make it compile in the new framework... However, after this modification, the beginEvent method is empty, and thus thread safe so code is working.

This is of course asking for a larger refactoring at one stage, but this requires to get rid of Run2 code first