Optimize GetFreePort(), bypass string manipulation

net.Listen("tcp", "127.0.0.1:0").Addr is a net.TCPAddr, use a type assertion
to get real value and then we have port.

Seems better, a little bit:

	go test -bench="BenchmarkGetFreePort*" ./util

	goos: linux
	goarch: amd64
	pkg: github.com/smallnest/rpcx/util
	cpu: Intel(R) Core(TM) i7-7500U CPU @ 2.70GHz
	BenchmarkGetFreePort_Old-4        132356              8869 ns/op
	BenchmarkGetFreePort_New-4        139147              8317 ns/op
	PASS
	ok      github.com/smallnest/rpcx/util  2.528s

I notice that GetFreePort() not used in rpcx anymore, is backword compatibility
the reason we must have it?

util/net.go

@@ -18,13 +18,7 @@ func GetFreePort() (port int, err error) {
 	}
 	defer listener.Close()
 
-	addr := listener.Addr().String()
-	_, portString, err := net.SplitHostPort(addr)
-	if err != nil {
-		return 0, err
-	}
-
-	return strconv.Atoi(portString)
+	return listener.Addr().(*net.TCPAddr).Port, nil
 }
 
 // ParseRpcxAddress parses rpcx address such as tcp@127.0.0.1:8972  quic@192.168.1.1:9981

util/net_test.go

@@ -1,6 +1,51 @@
 package util
 
-import "testing"
+import (
+	"net"
+	"strconv"
+	"testing"
+)
+
+func TestGetFreePort(t *testing.T) {
+	for i := 0; i < 1000; i++ {
+		port, err := GetFreePort()
+		if err != nil {
+			t.Error(err)
+		}
+
+		if port == 0 {
+			t.Error("GetFreePort() return 0")
+		}
+	}
+}
+
+var oldGetFreePort = func() (port int, err error) {
+	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		return 0, err
+	}
+	defer listener.Close()
+
+	addr := listener.Addr().String()
+	_, portString, err := net.SplitHostPort(addr)
+	if err != nil {
+		return 0, err
+	}
+
+	return strconv.Atoi(portString)
+}
+
+func BenchmarkGetFreePort_Old(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		oldGetFreePort()
+	}
+}
+
+func BenchmarkGetFreePort_New(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		GetFreePort()
+	}
+}
 
 func TestExternalIPV4(t *testing.T) {
 	ip, err := ExternalIPV4()