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/topology/collection.go

@@ -0,0 +1,57 @@
+package topology
+
+import (
+	"fmt"
+
+	"github.com/chrislusf/seaweedfs/weed/storage"
+	"github.com/chrislusf/seaweedfs/weed/util"
+)
+
+type Collection struct {
+	Name                     string
+	volumeSizeLimit          uint64
+	storageType2VolumeLayout *util.ConcurrentReadMap
+}
+
+func NewCollection(name string, volumeSizeLimit uint64) *Collection {
+	c := &Collection{Name: name, volumeSizeLimit: volumeSizeLimit}
+	c.storageType2VolumeLayout = util.NewConcurrentReadMap()
+	return c
+}
+
+func (c *Collection) String() string {
+	return fmt.Sprintf("Name:%s, volumeSizeLimit:%d, storageType2VolumeLayout:%v", c.Name, c.volumeSizeLimit, c.storageType2VolumeLayout)
+}
+
+func (c *Collection) GetOrCreateVolumeLayout(rp *storage.ReplicaPlacement, ttl *storage.TTL) *VolumeLayout {
+	keyString := rp.String()
+	if ttl != nil {
+		keyString += ttl.String()
+	}
+	vl := c.storageType2VolumeLayout.Get(keyString, func() interface{} {
+		return NewVolumeLayout(rp, ttl, c.volumeSizeLimit)
+	})
+	return vl.(*VolumeLayout)
+}
+
+func (c *Collection) Lookup(vid storage.VolumeId) []*DataNode {
+	for _, vl := range c.storageType2VolumeLayout.Items() {
+		if vl != nil {
+			if list := vl.(*VolumeLayout).Lookup(vid); list != nil {
+				return list
+			}
+		}
+	}
+	return nil
+}
+
+func (c *Collection) ListVolumeServers() (nodes []*DataNode) {
+	for _, vl := range c.storageType2VolumeLayout.Items() {
+		if vl != nil {
+			if list := vl.(*VolumeLayout).ListVolumeServers(); list != nil {
+				nodes = append(nodes, list...)
+			}
+		}
+	}
+	return
+}