HDFS-6513 Add MirrorJournalManager to transfer edits from primary cluster to Mirror cluster
HDFS-6514 Add MirrorJournal ( server side implementation) to handle the journals from Primary cluster
HDFS-6467 Format mirror cluster with primary cluster's NamespaceInfo in the runtime
HDFS-6483 Add Disaster Recovery related metrics
HDFS-6484 Disable APIs which may modify the mirror cluster