esp_mg_net_if refactoring: split LWIP & ESP parts

esp_mg_net_if was a rat's nest of mongoose net_if and event manager
implementaions and the event loop task for the ESP8266 miot port.
From that, pieces that are related to LWIP support have been pulled out,
separated into net_if and event manager files, and event loop task
has been moved under the miot esp8266 dirrectory.

This is done to facilitate LWIP code reuse. This may not be the end of
it, but it's a start.

Note: custom retransmit logic has been removed for now. Can be
reintroduced later is necessary.

PUBLISHED_FROM=fd5bbf75714583ce95776d4c76b6c5b5dc535364

mongoose.h

@@ -433,6 +433,15 @@ typedef struct stat cs_stat_t;
 unsigned long os_random(void);
 #define random os_random
 
+#ifndef RTOS_SDK
+#define MG_NET_IF_LWIP
+struct mg_mgr;
+struct mg_connection;
+uint32_t mg_lwip_get_poll_delay_ms(struct mg_mgr *mgr);
+void mg_lwip_set_keepalive_params(struct mg_connection *nc, int idle,
+                                  int interval, int count);
+#endif
+
 #endif /* CS_PLATFORM == CS_P_ESP_LWIP */
 #endif /* CS_COMMON_PLATFORMS_PLATFORM_ESP_LWIP_H_ */
 #ifdef MG_MODULE_LINES