Fixed WSF_FILE_RESPONSE and WSF_DOWNLOAD_RESPONSE
and set the status code to be Ok by default
Introduced WSF_ROUTER_SESSION

This fixes CQS violation from WSF_ROUTER.dispatch_and_return_handler (...): ? WSF_HANDLER
and related code, and this is more compliant with concurrency.

In addition, the WSF_ROUTER_SESSION can be enhanced in the future to answer more advanced needs.
Added HTTP_AUTHORIZATION.is_basic: BOOLEAN  query to know if this is a Basic HTTP Authorization.
Added WSF_HANDLER_FILTER_WRAPPER to build a bridge from router to filter.
Now WSF_FILTER_HANDLER is a handler and has formal generic G constrained to WSF_HANDLER
This eases implementation of potential descendants.
Fixed signature of `set_next' to allow redefinition.
Added assertions
cosmetic
Removed WSF_AGENT_HANDLER since it was an artificial notion,
as we have no common ancestor for WSF_HANDLER having
   execute (req: WSF_REQUEST; res: WSF_RESPONSE)
cosmetic