pkg/idutil: reduce conflict rate from 1% to 0.005%

Perviously, we only use 8bits from member identification
in id generation. The conflict rate is A(256,3)/256^3, which
is around 1%. Now we use 16bites to reduce the rate to 0.005%.

We can attach the full member id into id generation if needed...

etcdserver/server.go

@@ -358,7 +358,7 @@ func NewServer(cfg *ServerConfig) (*EtcdServer, error) {
 		lstats:        lstats,
 		SyncTicker:    time.Tick(500 * time.Millisecond),
 		peerRt:        prt,
-		reqIDGen:      idutil.NewGenerator(uint8(id), time.Now()),
+		reqIDGen:      idutil.NewGenerator(uint16(id), time.Now()),
 		forceVersionC: make(chan struct{}),
 		msgSnapC:      make(chan raftpb.Message, maxInFlightMsgSnap),
 	}