Persist Network Serial to Store to avoid outdated netmap sent (#260)

Fix outdated update coming from management
even when it is actually not outdated.

client/internal/engine.go

@@ -85,7 +85,7 @@ type Engine struct {
 	udpMuxConn      *net.UDPConn
 	udpMuxConnSrflx *net.UDPConn
 
-	// networkSerial is the latest Serial (state ID) of the network sent by the Management service
+	// networkSerial is the latest CurrentSerial (state ID) of the network sent by the Management service
 	networkSerial uint64
 }
 

client/internal/engine_test.go

@@ -96,7 +96,7 @@ func TestEngine_UpdateNetworkMap(t *testing.T) {
 		expectedSerial: 1,
 	}
 
-	// 2nd case - one extra peer added and network map has Serial grater than local => apply the update
+	// 2nd case - one extra peer added and network map has CurrentSerial grater than local => apply the update
 	case2 := testCase{
 		name: "input with an old peer and a new peer to add",
 		networkMap: &mgmtProto.NetworkMap{