Smooth async support

These changes basically keep source code compatibility for framework users. If some users have extensions that depend on LocaleFeature, RequestScopeFeature or SkinnySessionFilter, they may need to rewrite them a little bit to accept implicit parameter.

- Add FutureOpsFeature trait
- Mixin FutureOpsFeature to SkinnyControllerBase by default
- Add HttpServletRequest implicit param to LocaleFeature, RequestScopeFeature and SkinnySessionFilter methods

Supporting fully asynchronous controller is out of the scope of this pull request. I just tried a little but there're some issues which prevent providing easy-to-use APIs.