Initial merge of Networking TS compatibility.

Merged from chriskohlhoff/asio master branch as of commit
4a4d28b0d24c53236e229bd1b5f378c9964b1ebb.

example/cpp03/tutorial/timer2/timer.cpp

@@ -19,7 +19,7 @@ void print(const boost::system::error_code& /*e*/)
 
 int main()
 {
-  boost::asio::io_service io;
+  boost::asio::io_context io;
 
   boost::asio::deadline_timer t(io, boost::posix_time::seconds(5));
   t.async_wait(&print);