AthenaMPConfig: using single event dispatching if number of maxEvents is small.
This fixes the problem appearing in very short jobs when some workers do not get to process any events. In particular, it means that FileMetaDataCreatorTool::postExecute
may not be called and such jobs try to write FileMetaData from the input file.
Fixes ATEAM-882
Edited by Maciej Pawel Szymanski