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/github.com/modern-go/concurrent/go_below_19.go

@@ -0,0 +1,33 @@
+//+build !go1.9
+
+package concurrent
+
+import "sync"
+
+// Map implements a thread safe map for go version below 1.9 using mutex
+type Map struct {
+	lock sync.RWMutex
+	data map[interface{}]interface{}
+}
+
+// NewMap creates a thread safe map
+func NewMap() *Map {
+	return &Map{
+		data: make(map[interface{}]interface{}, 32),
+	}
+}
+
+// Load is same as sync.Map Load
+func (m *Map) Load(key interface{}) (elem interface{}, found bool) {
+	m.lock.RLock()
+	elem, found = m.data[key]
+	m.lock.RUnlock()
+	return
+}
+
+// Load is same as sync.Map Store
+func (m *Map) Store(key interface{}, elem interface{}) {
+	m.lock.Lock()
+	m.data[key] = elem
+	m.lock.Unlock()
+}