Fully enable ServerShutdownHandler after master joins the cluster

Once ROOT and META are assigned, ServerShutdownHandler is enabled. So that we can handle meta/root region server failure before joinCluster is completed. However, we can hold ServerShutdownHandler a little bit more for the user region assignments, i.e. doesn't assign user regions before joinCluster is returned. If so, we can avoid some region assignments racing: same regions are trying to be assigned in both joinCluster and ServerShutdownHandler.