Add basic_timeout_stream:

This stream wrapper replaces basic_stream_socket and
provides just the timeout functionality.

* basic_stream_socket is removed

test/beast/core/timeout_stream.cpp

@@ -0,0 +1,11 @@
+//
+// Copyright (c) 2018 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
+//
+
+// Test that header file is self-contained.
+#include <boost/beast/core/timeout_stream.hpp>