PerfMonComps : Start measuring Execute w/ the Start of AthOutSeq instead of AthRegSeq

Since we dropped AthRegSeq w/ !35299 (merged) we cannot use it as a hook to start the timer for the execute step in PerfMonMTSvc anymore. Therefore, we're switching to AthOutSeq instead. Eventually, we can try to figure out the hooks the service can/should use during configuration but this should do it for the time being.