fix(*.go): rename HeartbeatTimeout to HeartbeatInterval

While re-reading the etcd tuning guide I realized that perhaps it is a
bit of a misnomer to call this a "Timeout" since unlike the "Election
Timeout" there is no immediate consequence of it being missed.

Perhaps naming it interval would be better?
leader must stop all heartbeat routines before stepdown and truncate its log
Merge pull request #167 from xiangli-cmu/dev

more safety
Merge pull request #166 from philips/change-heartbeatTimeout-to-heartbeatInterval

fix(*.go): rename HeartbeatTimeout to HeartbeatInterval
Merge github.com:goraft/raft