Refactor all examples:

fix #575, fix #604, fix #608, fix #634, fix #712

All examples are rewritten:

* Using Best Practices
* Mostly self-contained
* New examples to complete the feature matrix
* The server-framework example is removed

example/http/client/coro/CMakeLists.txt

@@ -0,0 +1,17 @@
+#
+# Copyright (c) 2016-2017 Vinnie Falco (vinnie dot falco at gmail dot com)
+#
+# Distributed under the Boost Software License, Version 1.0. (See accompanying
+# file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+#
+# Official repository: https://github.com/boostorg/beast
+#
+
+GroupSources(include/boost/beast beast)
+GroupSources(example/http/client/coro "/")
+
+add_executable (http-client-coro
+    ${BOOST_BEAST_INCLUDES}
+    Jamfile
+    http_client_coro.cpp
+)

example/http/client/coro/Jamfile

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+#
+# Copyright (c) 2016-2017 Vinnie Falco (vinnie dot falco at gmail dot com)
+#
+# Distributed under the Boost Software License, Version 1.0. (See accompanying
+# file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+#
+# Official repository: https://github.com/boostorg/beast
+#
+
+exe http-client-coro :
+    http_client_coro.cpp
+    :
+    <variant>coverage:<build>no
+    <variant>ubasan:<build>no
+    ;

example/http/client/coro/http_client_coro.cpp

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+//
+// Copyright (c) 2016-2017 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/boostorg/beast
+//
+
+//------------------------------------------------------------------------------
+//
+// Example: HTTP client, coroutine
+//
+//------------------------------------------------------------------------------
+
+#include <boost/beast/core.hpp>
+#include <boost/beast/http.hpp>
+#include <boost/beast/version.hpp>
+#include <boost/asio/connect.hpp>
+#include <boost/asio/spawn.hpp>
+#include <boost/asio/ip/tcp.hpp>
+#include <cstdlib>
+#include <functional>
+#include <iostream>
+#include <string>
+
+using tcp = boost::asio::ip::tcp;       // from <boost/asio/ip/tcp.hpp>
+namespace http = boost::beast::http;    // from <boost/beast/http.hpp>
+
+//------------------------------------------------------------------------------
+
+// Report a failure
+void
+fail(boost::system::error_code ec, char const* what)
+{
+    std::cerr << what << ": " << ec.message() << "\n";
+}
+
+// Performs an HTTP GET and prints the response
+void
+do_session(
+    std::string const& host,
+    std::string const& port,
+    std::string const& target,
+    boost::asio::io_service& ios,
+    boost::asio::yield_context yield)
+{
+    boost::system::error_code ec;
+
+    // These objects perform our I/O
+    tcp::resolver resolver{ios};
+    tcp::socket socket{ios};
+
+    // Look up the domain name
+    auto const lookup = resolver.async_resolve({host, port}, yield[ec]);
+    if(ec)
+        return fail(ec, "resolve");
+
+    // Make the connection on the IP address we get from a lookup
+    boost::asio::async_connect(socket, lookup, yield[ec]);
+    if(ec)
+        return fail(ec, "connect");
+
+    // Set up an HTTP GET request message
+    http::request<http::string_body> req{http::verb::get, target, 11};
+    req.set(http::field::host, host);
+    req.set(http::field::user_agent, BOOST_BEAST_VERSION_STRING);
+
+    // Send the HTTP request to the remote host
+    http::async_write(socket, req, yield[ec]);
+    if(ec)
+        return fail(ec, "write");
+
+    // This buffer is used for reading and must be persisted
+    boost::beast::flat_buffer b;
+
+    // Declare a container to hold the response
+    http::response<http::dynamic_body> res;
+
+    // Receive the HTTP response
+    http::async_read(socket, b, res, yield[ec]);
+    if(ec)
+        return fail(ec, "read");
+
+    // Write the message to standard out
+    std::cout << res << std::endl;
+
+    // Gracefully close the socket
+    socket.shutdown(tcp::socket::shutdown_both, ec);
+
+    // not_connected happens sometimes
+    // so don't bother reporting it.
+    //
+    if(ec && ec != boost::system::errc::not_connected)
+        return fail(ec, "shutdown");
+
+    // If we get here then the connection is closed gracefully
+}
+
+//------------------------------------------------------------------------------
+
+int main(int argc, char** argv)
+{
+    // Check command line arguments.
+    if(argc != 4)
+    {
+        std::cerr <<
+            "Usage: http-client-coro <host> <port> <target>\n" <<
+            "Example:\n" <<
+            "    http-client-coro www.example.com 80 /\n";
+        return EXIT_FAILURE;
+    }
+    auto const host = argv[1];
+    auto const port = argv[2];
+    auto const target = argv[3];
+
+    // The io_service is required for all I/O
+    boost::asio::io_service ios;
+
+    // Launch the asynchronous operation
+    boost::asio::spawn(ios, std::bind(
+        &do_session,
+        std::string(host),
+        std::string(port),
+        std::string(target),
+        std::ref(ios),
+        std::placeholders::_1));
+
+    // Run the I/O service. The call will return when
+    // the get operation is complete.
+    ios.run();
+
+    return EXIT_SUCCESS;
+}