add move assignment operator
add move assignment operator

Reviewed By: vitaut

Differential Revision: D19401513

fbshipit-source-id: 6f9fa460490a695c1e8bb700ad89df3536657118

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift