optimize the read operation

[baotiao]

right now the read operation need Append Entry RPC, in raft's paper and paxos paper, both of them suggest leader lease to optimize the read operation. we can call it raft with lease.

[baotiao]

according to Diego's phd paper, there is two ways of optimize the read operation

1. use heartbeat instead of append entry RPC to avoid synchronous disk write
2. using clocks to reduce messaging, aka. leader with lease, but he didn't support this way since the time can't be believe in asynchronous network.

so we will implement the first way