Prevent spurious RX timeout/close on incoming socket object

When accepting a socket from an incoming connection, the internal fd was
assigned without updating the field for last RX activity. If the remote
side did *not* immediately write to the connection, or otherwise the
socket could not be internally checked for incoming data, AND the
higher-level code set up a RX timeout value, the 125-ms interval polling
might evaluate the socket RX timeout condition with a last RX activity
timestamp of zero (as per the default constructor). This caused a
spurious RX timeout and incorrect socket closing. Fixed by initializing
the field when the new incoming connection socket is accepted.

src/AsyncTCP.cpp

@@ -307,6 +307,7 @@ AsyncClient::AsyncClient(int sockfd)
         xSemaphoreTakeRecursive(_asyncsock_mutex, (TickType_t)portMAX_DELAY);
         _conn_state = 4;
         _socket = sockfd;
+        _rx_last_packet = millis();
         xSemaphoreGiveRecursive(_asyncsock_mutex);
     }
 }