Improve performance of TTree::Fill();

TTree::Fill() is a hotspot in the random number generation benchmark due to the condition checking if an auto-save or auto-flush action must be performed. Both checks use a "fEntries % N == 0" operation, which is as expensive as a division. This expensive operation can be replaced by a check for equality if we use a counter that is reset every time an auto-flush or auto-save is performed.