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/notification.go

@@ -26,8 +26,10 @@ import (
 	"net/url"
 	"path"
 	"sync"
+	"time"
 
 	"github.com/minio/minio/cmd/logger"
+	"github.com/minio/minio/pkg/auth"
 	"github.com/minio/minio/pkg/event"
 	"github.com/minio/minio/pkg/hash"
 	xnet "github.com/minio/minio/pkg/net"
@@ -90,6 +92,33 @@ func (sys *NotificationSys) DeleteBucket(bucketName string) <-chan NotificationP
 	return errCh
 }
 
+// SetCredentials - calls SetCredentials RPC call on all peers.
+func (sys *NotificationSys) SetCredentials(credentials auth.Credentials) map[xnet.Host]error {
+	errors := make(map[xnet.Host]error)
+	var wg sync.WaitGroup
+	for addr, client := range sys.peerRPCClientMap {
+		wg.Add(1)
+		go func(addr xnet.Host, client *PeerRPCClient) {
+			defer wg.Done()
+			// Try to set credentials in three attempts.
+			for i := 0; i < 3; i++ {
+				err := client.SetCredentials(credentials)
+				if err == nil {
+					break
+				}
+				errors[addr] = err
+				// Wait for one second and no need wait after last attempt.
+				if i < 2 {
+					time.Sleep(1 * time.Second)
+				}
+			}
+		}(addr, client)
+	}
+	wg.Wait()
+
+	return errors
+}
+
 // SetBucketPolicy - calls SetBucketPolicy RPC call on all peers.
 func (sys *NotificationSys) SetBucketPolicy(bucketName string, bucketPolicy *policy.Policy) <-chan NotificationPeerErr {
 	errCh := make(chan NotificationPeerErr)