Fix data race in http server examples

When using `beast::tcp_stream`, the user must make sure that async operations
are initiated from within the strand associated with the stream.

Signed-off-by: Damian Jarek <damian.jarek93@gmail.com>

example/http/server/async/http_server_async.cpp

@@ -16,6 +16,7 @@
 #include <boost/beast/core.hpp>
 #include <boost/beast/http.hpp>
 #include <boost/beast/version.hpp>
+#include <boost/asio/dispatch.hpp>
 #include <boost/asio/strand.hpp>
 #include <boost/config.hpp>
 #include <algorithm>
@@ -273,7 +274,14 @@ public:
     void
     run()
     {
-        do_read();
+        // We need to be executing within a strand to perform async operations
+        // on the I/O objects in this session. Although not strictly necessary
+        // for single-threaded contexts, this example code is written to be
+        // thread-safe by default.
+        net::dispatch(stream_.get_executor(),
+                      beast::bind_front_handler(
+                          &session::do_read,
+                          shared_from_this()));
     }
 
     void