net::ssl::stream is canonical in examples

We no longer need to use beast::ssl_stream to improve write performance, as
Asio now linearizes scatter/gather I/O in ssl::stream:
https://github.com/chriskohlhoff/asio/commit/17637a48ccbfa2f63941d8393a7c8316a8df4a79

example/http/server/stackless-ssl/http_server_stackless_ssl.cpp

@@ -17,10 +17,10 @@
 
 #include <boost/beast/core.hpp>
 #include <boost/beast/http.hpp>
-#include <boost/beast/ssl.hpp>
 #include <boost/beast/version.hpp>
 #include <boost/asio/coroutine.hpp>
 #include <boost/asio/dispatch.hpp>
+#include <boost/asio/ssl.hpp>
 #include <boost/asio/strand.hpp>
 #include <boost/config.hpp>
 #include <algorithm>
@@ -239,7 +239,7 @@ class session
     : public boost::asio::coroutine
     , public std::enable_shared_from_this<session>
 {
-    beast::ssl_stream<beast::tcp_stream> stream_;
+    ssl::stream<beast::tcp_stream> stream_;
     beast::flat_buffer buffer_;
     std::shared_ptr<std::string const> doc_root_;
     http::request<http::string_body> req_;