HltEventLoopMgr: Use GenericMonitoringTool and add total time monitoring

There are two changes made in the HLT event loop manager (separate commits):

1. Use GenericMonitoringTool instead of a direct handle to THistSvc. This was only possible after recent updates which allowed for creating 2D string vs string histograms with the Monitored infrastructure.
2. Implement histograms with total event processing time for all events, accepted events and rejected events, as we used to have in Run 2 (ATR-19667)