Unify MainServicesSerialCfg and MainServicesThreadedCfg

Walter Lamplrequested to merge
wlampl/athena:MainServicesCfgUpdate into master

This MR unifies the MainServicesSerialCfg and MainServicesThreadedCfg methods into a single MainServicesCfg(ConfigFlags) method. Config-flags decide if the job runs in MT mode or serial mode.

On top: Introduce a few new config-flags: Exec.MaxEvents, Exec.SkipEvents, Exec.OutputLevel and Exec.DebugStage.

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