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:

https://github.com/NetSys/sts2-applications/commit/687a244aa2b2a94b05337d62302fe2a8429c78e1 https://github.com/NetSys/sts2-applications/commit/6c81d34108bb28d59ff25eeb26b4a2b469bc310c