fix name shadowing issue (#123)

fix compiling error: 
> error: declaration of ‘initialized’ shadows a member of 'this'
Signed-off-by: tabokie <xy.tao@outlook.com>

src/db_impl.h

@@ -136,7 +136,7 @@ class TitanDBImpl : public TitanDB {
 
   void StartBackgroundTasks();
 
-  void TEST_set_initialized(bool initialized) { initialized_ = initialized; }
+  void TEST_set_initialized(bool _initialized) { initialized_ = _initialized; }
 
   Status TEST_StartGC(uint32_t column_family_id);
   Status TEST_PurgeObsoleteFiles();