Make EventCounterAlg reentrant

Motivated by @ishapova's presentation at yesterday's Core Software meeting. The updated code is (was) essentially a copy of the Gaudi's [EventCounter]((https://gitlab.cern.ch/gaudi/Gaudi/-/blob/199b5287a7e024ab7ac11556ebaca2f9014dfd7f/GaudiAlg/src/lib/EventCounter.cpp).