Revert vendor directory to support old golang (GOPATH mode)

gomodule is still in progress to migrate for now, hence multus
team decide to keep vendor directory to support build without
gomodule.

vendor/google.golang.org/grpc/internal/grpcrand/grpcrand.go

@@ -0,0 +1,56 @@
+/*
+ *
+ * Copyright 2018 gRPC authors.
+ *
+ * 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 grpcrand implements math/rand functions in a concurrent-safe way
+// with a global random source, independent of math/rand's global source.
+package grpcrand
+
+import (
+	"math/rand"
+	"sync"
+	"time"
+)
+
+var (
+	r  = rand.New(rand.NewSource(time.Now().UnixNano()))
+	mu sync.Mutex
+)
+
+// Int63n implements rand.Int63n on the grpcrand global source.
+func Int63n(n int64) int64 {
+	mu.Lock()
+	res := r.Int63n(n)
+	mu.Unlock()
+	return res
+}
+
+// Intn implements rand.Intn on the grpcrand global source.
+func Intn(n int) int {
+	mu.Lock()
+	res := r.Intn(n)
+	mu.Unlock()
+	return res
+}
+
+// Float64 implements rand.Float64 on the grpcrand global source.
+func Float64() float64 {
+	mu.Lock()
+	res := r.Float64()
+	mu.Unlock()
+	return res
+}