Design finalize http.sys high level api

[youyuanwu]

http_connection, winnet::http::simple_request and winnet::http::simple_response are exposed for ease of use for a server application.
Currently http server to user logic is handled by a handler function:

winasio/tests/http/http_server_test.cpp

Line 47 in 3ec6862

auto handler = [](const winnet::http::simple_request &request,

We need to improve and finalize this API.

One example of high level API is this: https://github.com/nghttp2/nghttp2-asio
https://www.nghttp2.org/documentation/libnghttp2_asio.html