Since esp32 has much more memory than esp31B, modify the HEAP_HIGHWAT from
6K to 20K.
Notes: HEAP_HIGHWAT is used for tcp/udp tx flow control, if socket task detects
that the free heap memory is less than HEAP_HIGHWAT, it stops to trasmitting until
the free heap memory is greater than HEAP_HIGHWAT
2. Add struct station_list into tcpip_adapter layer
3. When ap -L cmd is received, get mac address from mac layer first and then search ip address based on mac address on dhcp layer.