EC: ReplicationManager - refactor logic to send datanode commands into a central place

The logic to send datanode commands, such as replicate, reconstruct, delete and close container happens in a few different places in the RM classes. It would make sense to centralise this in some common code, as it will make it more re-usable. There is some logic in the balancer which could re-use these new central methods to send replicate and delete commands too.