Implement HTTP POST based RPC (#5840)

Added support for new RPC support using HTTP POST.  RPC's 
arguments and reply are Gob encoded and sent as HTTP 
request/response body.

This patch also removes Go RPC based implementation.

cmd/rpc/client_test.go

@@ -0,0 +1,72 @@
+/*
+ * Minio Cloud Storage, (C) 2018 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package rpc
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	xnet "github.com/minio/minio/pkg/net"
+)
+
+func TestClientCall(t *testing.T) {
+	rpcServer := NewServer()
+	if err := rpcServer.RegisterName("Arith", &Arith{}); err != nil {
+		t.Fatalf("unexpected error %v", err)
+	}
+
+	httpServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		rpcServer.ServeHTTP(w, r)
+	}))
+	defer httpServer.Close()
+
+	url, err := xnet.ParseURL(httpServer.URL)
+	if err != nil {
+		t.Fatalf("unexpected error %v", err)
+	}
+	rpcClient := NewClient(url, nil, DefaultRPCTimeout)
+
+	var reply int
+	var boolReply bool
+	var intArg int
+
+	testCases := []struct {
+		serviceMethod string
+		args          interface{}
+		reply         interface{}
+		expectErr     bool
+	}{
+		{"Arith.Multiply", Args{7, 8}, &reply, false},
+		{"Arith.Multiply", &Args{7, 8}, &reply, false},
+		// rpc reply must be a pointer type but found int error.
+		{"Arith.Multiply", &Args{7, 8}, reply, true},
+		// gob: type mismatch in decoder: want struct type rpc.Args; got non-struct error.
+		{"Arith.Multiply", intArg, &reply, true},
+		// gob: decoding into local type *bool, received remote type int error.
+		{"Arith.Multiply", &Args{7, 8}, &boolReply, true},
+	}
+
+	for i, testCase := range testCases {
+		err := rpcClient.Call(testCase.serviceMethod, testCase.args, testCase.reply)
+		expectErr := (err != nil)
+
+		if expectErr != testCase.expectErr {
+			t.Fatalf("case %v: expected: %v, got: %v", i+1, testCase.expectErr, expectErr)
+		}
+	}
+}