EC: Notify ReplicationManager when a heartbeat updates datanode command counts

When a datanode heartbeat is processed, it updates the queued command counts on the datanodes. Replication is going to use this information when assigning work to datanodes, and if a datanode exceeds the load limit it will be excluded until it has more capacity.

To allow RM to remove nodes from any exclude lists, we should fire a SCM event when the counts are updated, as this will avoid RM polling frequently.

For now, the notification in RM does not do anything - we will add code which uses it in future changes as we build out the load aware under / over replication processing.