raft: add handleHeartbeat
handleHeartbeat commits to the commit index in the message. It never decreases the
commit index of the raft state machine.
Merge pull request #1739 from yichengq/230

rafthttp: send takes raft message instead of bytes
Merge pull request #1740 from xiang90/handleheartbeat

raft: add handleHeartbeat
rafthttp: streaming server and client
change