Make AsyncClient noncopyable/nonmovable

The previous code tried to implement move-as-copy semantics, which can
lead to unexpected behavior; and further it was not safe, as there was a
risk of race conditions with the LwIP thread.

Remove the unsafe function for now, and indicate to the compiler to
check for it.

If there's a strong need for a move operation in the future, the thread
safety can be reviewed at that time.

Author: willmmiles

src/AsyncTCP.cpp

@@ -759,24 +759,6 @@ AsyncClient::~AsyncClient() {
  * Operators
  * */
 
-AsyncClient &AsyncClient::operator=(const AsyncClient &other) {
-  if (_pcb) {
-    _close();
-  }
-
-  _pcb = other._pcb;
-  _closed_slot = other._closed_slot;
-  if (_pcb) {
-    _rx_last_packet = millis();
-    tcp_arg(_pcb, this);
-    tcp_recv(_pcb, &_tcp_recv);
-    tcp_sent(_pcb, &_tcp_sent);
-    tcp_err(_pcb, &_tcp_error);
-    tcp_poll(_pcb, &_tcp_poll, CONFIG_ASYNC_TCP_POLL_TIMER);
-  }
-  return *this;
-}
-
 bool AsyncClient::operator==(const AsyncClient &other) const {
   return _pcb == other._pcb;
 }

src/AsyncTCP.h

@@ -80,7 +80,13 @@ class AsyncClient {
   AsyncClient(tcp_pcb *pcb = 0);
   ~AsyncClient();
 
-  AsyncClient &operator=(const AsyncClient &other);
+  // Noncopyable
+  AsyncClient(const AsyncClient &) = delete;
+  AsyncClient &operator=(const AsyncClient &) = delete;
+
+  // Nonmovable
+  AsyncClient(AsyncClient &&) = delete;
+  AsyncClient &operator=(AsyncClient &&) = delete;
 
   bool operator==(const AsyncClient &other) const;