Correct lease with check quorum.

[Hoverbear]

Fixes #140 .

You can see etcd also has this behavior: https://github.com/etcd-io/etcd/blob/83304cfc808cf6303d48c45a696f169fae422e68/raft/raft.go#L1010-L1037

	case pb.MsgReadIndex:
		if r.quorum() > 1 {
			if r.raftLog.zeroTermOnErrCompacted(r.raftLog.term(r.raftLog.committed)) != r.Term {
				// Reject read only request when this leader has not committed any log entry at its term.
				return nil
			}

			// thinking: use an interally defined context instead of the user given context.
			// We can express this in terms of the term and index instead of a user-supplied value.
			// This would allow multiple reads to piggyback on the same message.
			switch r.readOnly.option {
			case ReadOnlySafe:
				r.readOnly.addRequest(r.raftLog.committed, m)
				r.bcastHeartbeatWithCtx(m.Entries[0].Data)
			case ReadOnlyLeaseBased:
				ri := r.raftLog.committed
				if m.From == None || m.From == r.id { // from local member
					r.readStates = append(r.readStates, ReadState{Index: r.raftLog.committed, RequestCtx: m.Entries[0].Data})
				} else {
					r.send(pb.Message{To: m.From, Type: pb.MsgReadIndexResp, Index: ri, Entries: m.Entries})
				}
			}
		} else {
			r.readStates = append(r.readStates, ReadState{Index: r.raftLog.committed, RequestCtx: m.Entries[0].Data})
		}

		return nil
}

[siddontang]

PTAL @xiang90

[siddontang]

LGTM

[Hoverbear]

Oh, @siddontang Please give a real LGTM (a review instead of a comment). :) bors won't merge since only one review.

[nolouch]

@Hoverbear Should we check the CheckQurom is enabled in the LeaseBase request? like: etcd-io/etcd@9801fd7

[Hoverbear]

@nolouch I think you're right! I'll reflect that here.

[Hoverbear]

@nolouch PTAL again! :)

[nolouch]

LGTM

[Hoverbear]

@siddontang ping, you said LGTM but did not actually approve it.

[Hoverbear]

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