clean backend

CMakeLists.txt

 cmake_minimum_required(VERSION 3.2)
 project(VisualDL)
 
+find_program(CCACHE_FOUND ccache)
+if(CCACHE_FOUND)
+  set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ccache)
+  set_property(GLOBAL PROPERTY RULE_LAUNCH_LINK ccache)
+endif(CCACHE_FOUND)
+
 set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
 set(CMAKE_CXX_STANDARD 11)
 set(CMAKE_CXX_FLAGS "-fPIC")

visualdl/storage/storage.cc

 #include <glog/logging.h>
-#include <visualdl/backend/utils/concurrency.h>
 #include <fstream>
 
-#include "visualdl/backend/storage/storage.h"
-#include "visualdl/backend/utils/filesystem.h"
+#include "visualdl/storage/storage.h"
+#include "visualdl/utils/concurrency.h"
+#include "visualdl/utils/filesystem.h"
 
 namespace visualdl {
 

visualdl/storage/storage_test.cc

-#include "visualdl/backend/storage/storage.h"
+#include "visualdl/storage/storage.h"
 
 #include <glog/logging.h>
 #include <gtest/gtest.h>