simplify RestartListener.Default using getBusyExecutors()

computeIdleExecutors includes connecting slaves, that seems to be the root cause of a bug I'm trying to fix : overalLoadStatistics repport more idle executors than total, so safe restart don't wait for builds to complete.
isReadyToRestart seems unecessary complex to compute idle vs total considering there's already a ComputerSet method to return the busy executor count.