coreos/etcd — pull request #1200 by xiangli-cmu (2014-09-29 17:47:06+00:00)
Title: raft: heartbeat is only response for maintaining leader dominance

Separate the health checking logic and actual entry appending logic.

Follower will always reply `accept` to heartbeat. Heartbeat will not trigger `msgApp` or `msgSnap` even if the follower is not synced with leader or is not matched with leader's log.