Before LwIP was moved into a submodule, sntp and esp_ping were
available as the following includes:
#include "apps/sntp/sntp.h"
#include "esp_ping.h"
This commit makes header files available at the old location, and adds
a deprecation warning for sntp.h.
Closes https://github.com/espressif/esp-idf/issues/2392
Do some refactor to socket ping:
1. Add ping_deinit to kill ping thread
2. Apply configuration set by esp_ping_set_target to socket ping
3. Limit only one ping thread
