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
Implement lwip per-thread-sem with:
vTaskSetThreadLocalStoragePointerAndDelCallback/pvTaskGetThreadLocalStoragePointer
register sys_thread_tls_free to free per-thread-sem when the thread is terminated.
Also let lwip use index CONFIG_LWIP_THREAD_LOCAL_STORAGE_INDEX
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.
The thread-local-storage feature in FreeRTOS attaches an application-usable array of pointers to a thread control block. These pointers usually point to a structure the thread allocates. When a thread gets (voluntarily or involuntarily) destroyed, this memory can leak. This merge adds a matching second array of user-settable pointers to destructor routines. As soon as the task gets cleaned up (which happens in the idle thread), the destructors get called and the memory can be freed.
See merge request !19
