Fix/3.x/2981

Provides new event on EntryListener, onMerge.

Previously in a WAN Replication scenario between 2 clusters, a MapEntry would travel to a destination cluster but the EntryListener would not fire there. So there was no way anybody listening in the destination cluster would know a new MapEntry had arrived.

This changes now fires the onMerge event in the EntryListener in the destination cluster.