WL#5820: Separate replication plugin interfaces from core plugin interfaces

Affects: Server-Prototype Only   —   Status: Un-Assigned   —   Priority: Medium

This is a step towards making replication a library, such that the 5.6 rpl-
lib can be plugged into the 5.5 core [WL#5675].

In this worklog, we separate the generic and core-specific interfaces from the 
replication-specific interfaces in rpl_handler.h and replication.h.