directory structure change to work with glide

glide has its own requirements. My previous workaround caused me some
code checkin errors. Need to fix this.

weed/util/concurrent_read_map.go

@@ -0,0 +1,60 @@
+package util
+
+import (
+	"sync"
+)
+
+// A mostly for read map, which can thread-safely
+// initialize the map entries.
+type ConcurrentReadMap struct {
+	sync.RWMutex
+
+	items map[string]interface{}
+}
+
+func NewConcurrentReadMap() *ConcurrentReadMap {
+	return &ConcurrentReadMap{items: make(map[string]interface{})}
+}
+
+func (m *ConcurrentReadMap) initMapEntry(key string, newEntry func() interface{}) (value interface{}) {
+	m.Lock()
+	defer m.Unlock()
+	if value, ok := m.items[key]; ok {
+		return value
+	}
+	value = newEntry()
+	m.items[key] = value
+	return value
+}
+
+func (m *ConcurrentReadMap) Get(key string, newEntry func() interface{}) interface{} {
+	m.RLock()
+	if value, ok := m.items[key]; ok {
+		m.RUnlock()
+		return value
+	}
+	m.RUnlock()
+	return m.initMapEntry(key, newEntry)
+}
+
+func (m *ConcurrentReadMap) Find(key string) (interface{}, bool) {
+	m.RLock()
+	value, ok := m.items[key]
+	m.RUnlock()
+	return value, ok
+}
+
+func (m *ConcurrentReadMap) Items() (itemsCopy []interface{}) {
+	m.RLock()
+	for _, i := range m.items {
+		itemsCopy = append(itemsCopy, i)
+	}
+	m.RUnlock()
+	return itemsCopy
+}
+
+func (m *ConcurrentReadMap) Delete(key string) {
+	m.Lock()
+	delete(m.items, key)
+	m.Unlock()
+}