Clustering : Make heartbeat messages higher priority

The RaftActor#Leader needs to more reliably deliver Heartbeat messages to it's Followers.
To ensure this we need to either make the SendHeartbeat message higher priority and use a priority mailbox to ensure that it gets scheduled ahead of all the other messages. Alternately we could also use a separate co-actor to send the heartbeats to the followers.

The goal is to ensure that in stable state elections do not get retriggered.