Clustering : RaftActor should always use a scheduled heartbeat

The behavior of the RaftActor#Leader is that when a message comes in it reschedules the heartbeat regardless of what the message is. This can lead to a slow follower possibly being starved in a situation where a fast follower keeps sending an AppendEntriesReply. In this situation the fast follower will keep getting more AppendEntries whereas the slow follower will not because the heartbeat mechanism which is supposed to send the AppendEntries to the second follower keeps getting rescheduled.

One possible fix for this is to simply keep a scheduled heartbeat regardless of other messages that may come in.