raft: leader waits for the reply of previous message when follower is not in good path.

It is reasonable for the leader to wait for the reply before sending out the next
msgApp or msgSnap for the follower in bad path. Or the leader will send out useless
messages if the previous message is rejected or the previous message is a snapshot.
Especially for the snapshot case, the leader will be 100% to send out duplicate message
including the snapshot, which is a huge waste.

This commit implement a timeout based wait mechanism. The timeout for normal msgApp is a
heartbeatTimeout and the timeout for snapshot is electionTimeout(snapshot is larger). We
can implement a piggyback mechanism(application notifies the msg lost) in the future
if necessary.
Merge pull request #1962 from xiang90/raft

raft: leader waits for the reply of previous message