(server) attempt at fixing #131 by using blocking writes in server mode

ixwebsocket/IXWebSocketTransport.h

@@ -61,7 +61,8 @@ namespace ix
         enum class PollResult
         {
             Succeeded,
-            AbnormalClose
+            AbnormalClose,
+            CannotFlushSendBuffer
         };
 
         using OnMessageCallback =
@@ -135,6 +136,10 @@ namespace ix
         // client should mask but server should not
         std::atomic<bool> _useMask;
 
+        // Tells whether we should flush the send buffer before
+        // saying that a send is complete. This is the mode for server code.
+        std::atomic<bool> _blockingSend;
+
         // Buffer for reading from our socket. That buffer is never resized.
         std::vector<uint8_t> _readbuf;
 
@@ -238,6 +243,7 @@ namespace ix
                                                size_t closeWireSize,
                                                bool remote);
 
+        bool flushSendBuffer();
         void sendOnSocket();
         WebSocketSendInfo sendData(wsheader_type::opcode_type type,
                                    const std::string& message,