add btree for volume index

weed/storage/needle/needle_value_map.go

@@ -0,0 +1,8 @@
+package needle
+
+type NeedleValueMap interface {
+	Set(key Key, offset, size uint32) (oldOffset, oldSize uint32)
+	Delete(key Key) uint32
+	Get(key Key) (*NeedleValue, bool)
+	Visit(visit func(NeedleValue) error) error
+}