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/peer-rpc-client-target.go

@@ -0,0 +1,52 @@
+/*
+ * 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 cmd
+
+import "github.com/minio/minio/pkg/event"
+
+// PeerRPCClientTarget - RPCClient is an event.Target which sends event to target of remote peer.
+type PeerRPCClientTarget struct {
+	id             event.TargetID
+	remoteTargetID event.TargetID
+	rpcClient      *PeerRPCClient
+	bucketName     string
+}
+
+// ID - returns target ID.
+func (target *PeerRPCClientTarget) ID() event.TargetID {
+	return target.id
+}
+
+// Send - sends event to remote peer by making RPC call.
+func (target *PeerRPCClientTarget) Send(eventData event.Event) error {
+	return target.rpcClient.SendEvent(target.bucketName, target.id, target.remoteTargetID, eventData)
+}
+
+// Close - does nothing and available for interface compatibility.
+func (target *PeerRPCClientTarget) Close() error {
+	return nil
+}
+
+// NewPeerRPCClientTarget - creates RPCClient target with given target ID available in remote peer.
+func NewPeerRPCClientTarget(bucketName string, targetID event.TargetID, rpcClient *PeerRPCClient) *PeerRPCClientTarget {
+	return &PeerRPCClientTarget{
+		id:             event.TargetID{targetID.ID, targetID.Name + "+" + mustGetUUID()},
+		remoteTargetID: targetID,
+		bucketName:     bucketName,
+		rpcClient:      rpcClient,
+	}
+}