Remove requirement for custom RPCClient (#5405)

This change is a simplification over existing
code since it is not required to have a separate
RPCClient structure instead keep authRPCClient can
do the same job.

There is no code which directly uses netRPCClient(),
keeping authRPCClient is better and simpler. This
simplication also allows for removal of multiple
levels of locking code per object.

Observed in #5160

cmd/s3-peer-rpc-handlers_test.go

@@ -62,9 +62,15 @@ func TestS3PeerRPC(t *testing.T) {
 // Test S3 RPC handlers
 func (s *TestRPCS3PeerSuite) testS3PeerRPC(t *testing.T) {
 	// Validate for invalid token.
-	args := AuthRPCArgs{AuthToken: "garbage"}
-	rclient := newRPCClient(s.testAuthConf.serverAddr, s.testAuthConf.serviceEndpoint, false)
+	args := AuthRPCArgs{}
+	rclient := newAuthRPCClient(s.testAuthConf)
 	defer rclient.Close()
+
+	if err := rclient.Login(); err != nil {
+		t.Fatal(err)
+	}
+
+	rclient.authToken = "garbage"
 	err := rclient.Call("S3.SetBucketNotificationPeer", &args, &AuthRPCReply{})
 	if err != nil {
 		if err.Error() != errInvalidToken.Error() {