DataTreeEventCallbackRegistrar does not work in Cluster in all scenarios

In a 3 node cluster, if event A is received on node 1, and if a DataTreeEventCallback is registered at node 1, for a DataTree whose shard leader is node2, the DataTreeEvent will get fired on node2, where the callback is not available. In such scenarios, the callback handling is buggy. We have to add a clustering knob to DataTreeEventCallbackRegistrar.