Follower does not commit entries on receiving heartbeat

[schuster]

If no new requests come in for a long time, it might be that the only way a follower knows to commit a request is to do so on a heartbeat message, so the commit logic should happen there, too.

Really, there should be no difference in the processing for heartbeats and non-heartbeats. A heartbeat is just an AppendEntries in which the list of new entries is empty, so an empty list is appended to the log.

[colin-scott]

For what it's worth, I have (non-pull-request-worthy) fix for this issue here:

NetSys/demi-applications@687a244
NetSys/demi-applications@6c81d34