Clustering : When it is detected that a follower is too far behind send it a snapshot instead of append entries

This is an optimization to get a slow follower to catch up a little faster.

The basic idea is as follows. Let's say we have a Follower which is about n entries behind the Leader and those n entries happen to have a dataSize which is greater than the size of the state snapshot. Then it would be more optimal to send the Follower a snapshot than to send it the n append entries (either 1 by 1 or even in batches)