HiveEventLoopMgr: fully process first event before scheduling any others

In order not to overload resources that get accessed during the first event (db lookups, conditions, etc), when running in MT, fully process the first event before scheduling any others.

This is controlled by the bool property HiveEventLoopMgr.FirstEventAlone which is by default True. If it is set False, then the scheduler will immediately schedule as many events as it deems necessary.