Fixes and simplifications to HTTP example server:

The example HTTP server is updated to provide the correct MIME-type.
It no longer uses the now-deprecated http::stream class, since that
implementation does not provide flow control. A new example async_write
function is provided in the asynchronous server for managing the
lifetime of a message sent asynchronously.

The logging is thread-safe, and a bug causing connections to
malfunction is fixed.

examples/http_server.cpp

@@ -57,8 +57,13 @@ int main(int ac, char const* av[])
     endpoint_type ep{address_type::from_string(ip), port};
 
     if(sync)
+    {
         http_sync_server server(ep, root);
+        beast::test::sig_wait();
+    }
     else
+    {
         http_async_server server(ep, threads, root);
-    beast::test::sig_wait();
+        beast::test::sig_wait();
+    }
 }