Checking by the send function that at least two maximum size Ethernet frames fit in the remaining heap before the LwIP packet buffer is allocated seems to increase stability. This can be caused by the fact that WLAN hardware interrupts allocate additional memory when receiving a frame during the send attempt.
The situation where the firmware `lwIP` packet buffer is exhausted is an important indication that the traffic sent to and sent from the esp8266 is more than the esp8266 is able to handle. Therefore, it should be an error message.
It is not necessary to realize timeout handling in send function or to disconnect from AP if lwIP packet buffer is exhausted. Waiting that the frame allocated in lwIP packet buffer is freed by MAC layer led to the complete blockage of send function on heavy network load. Disconnecting from AP is counterproductive since reconnecting usually fails on heavy network load.
Disconnecting from the AP in the send function if the lwIP packet buffer is exhausted is counterproductive since reconnecting usually fails on heavy network load. A better strategy is to slow down the sending of MAC frames from netif a bit to wait for flushing the buffer in the MAC layer.
Since _esp_wifi_recv_cb is not executed in interrupt context but in the context of the `ets` thread, it is not necessary to pass the`NETDEV_EVENT_ISR` event first. Instead, the receive function can be called directly which result in much faster handling, a less frame lost rate and more robustness.
Since _esp_wifi_recv_cb is not executed in interrupt context but in the context of the `ets` thread, the receive function can be called directly. There is no need for a mutex anymore to synchronize the access to the receive buffer between _esp_wifi_recv_cb and _recv function.
When the size of a received frame is checked, always the total length should be used instead of the length of the first lwIP pbuf in the pbuf chain. Otherwise, the check that the length does not exceed ETHERNET_MAX_LEN will always be true since the maximum size of one lwIP pbuf in a pbuf chain is 512 bytes.
Although only the station interface is needed, the WiFi interface has to be used in SoftAP + Station mode. Otherwise the send function blocks sporadically. Since the SoftAP interface is not used, it is configured with a hidden SSID and a long beacon interval. Connections from other stations are not allowed.
Fixes sporadic blocking of the wifi thread in esp_wifi_recv_cb function under heavy network load conditions when frames are coming in faster than they can be processed. Since esp_wifi_recv_cb function is not executed in interrupt context, the msg_send function used for ISR event can block when the message queue is full. With this change esp_wifi can be flooded with icmpv6 packets of maximum size without any problems over hours.
