Master fails to startup on large clusters, refreshing block distribution

On our large clusters with, master has failed to startup within specified time and aborted itself since it was initializing HDFS block distribution. Enable table also takes time for larger tables for the same reason. My proposal is to refresh HDFS block distribution at the end of master initialization and not at retainAssignment()'s createCluster(). This would address HBASE-16570's intention, but avoid the problems we ran into.

cc aoxiang tedyu