With Raft not all append entries are expected to reach the followers
so it is a requirement that when acknowledgement of an append entries
is not received we send a duplicate append entries.

In replication scenarios where a lot of rapid replication messages
are being generated the lack of equally rapid acknowledgement results
in a lot of duplicate messages being generated. This later results in
a cycle when the replies for those duplicate replication messages start
arriving and result in even more duplicate replication messages being
sent.

We need to disallow a replication message
to be generated for a follower if the nextIndex for that follower
has not changed and the time since the last replication message
does not exceed the heartbeat interval.