Protect write buffer queue with a per-client mutex

This prevents a race condicion of the ::add() method modifying the queue
and being preempted by the high-priority asyncTcpSock task that in turn
flushes the same queue to the socket, therefore modifying the queue.

src/AsyncTCP.h

@@ -180,6 +180,7 @@ class AsyncClient : public AsyncSocketBase
     } queued_writebuf;
 
     // Queue of buffers to write to socket
+    SemaphoreHandle_t _write_mutex;
     std::deque<queued_writebuf> _writeQueue;
     bool _ack_timeout_signaled = false;