raft: leader waits for the reply of previous message 

It is reasonable for the leader to wait for the reply before sending out the next
msgApp or msgSnap when the follower is 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.

Fix #1849 

@bdarnell @yichengq  I am adding test for this.
So shouldWait is only for the initial startup, and once a node has been caught up we will always send messages? OK. I was expecting to go into wait mode any time the node reported being behind.