1. Added API to create write_concerns
2. Added accessor apis for write_concerns
3. Added cast to remove compilation hint
4. Removed logic that prevented write_concern = 1 from being embedded in bson cmd object
1. Added API for creating write_concerns
2. Added accesor apis for write_concerns
3. Removed warning because of lack of cast
4. Removed logic that prevented w=1 in writeconcern from being embedded on cmd bson object