Add attribute to model which will indicate that backup is synchronized with live server

If HA is configured with replicated journal then it takes some time to backup to synchronize with live server. Once backup is in sync with live then following information appears in server.log:

13:20:00,739 INFO  [org.apache.activemq.artemis.core.server] (Thread-3 (ActiveMQ-client-netty-threads-457000966)) AMQ221024: Backup server ActiveMQServerImpl::serverUUID=bc015b34-fd73-11e5-80ca-1b35f669abb8 is synchronized with live-server.
13:20:01,500 INFO  [org.apache.activemq.artemis.core.server] (Thread-2 (ActiveMQ-server-org.apache.activemq.artemis.core.server.impl.ActiveMQServerImpl$2@41f992ab-83559664)) AMQ221031: backup announced

Reading server logs to see whether backup is in sync is not convinient and user friendly way. Could we add this information to model so it can be detected through CLI command?

New attribute with name synchronized-with-live should be added to /subsystem=messaging-activemq/server=default/ha-policy=replication-slave

This information can be gathered by calling: SharedNothingBackupActivation.isRemoteBackupUpToDate()

Additionally we should provide this attribute also in replicated live server in: /subsystem=messaging-activemq/server=default/ha-policy=replication-master as Administrator might want to know whether live can be shutdown.