Update for ESP8266 Arduino Core main branch HEAD and next release

Beginning with esp8266/Arduino@c7c7d05, the development versions of ESP8266 Arduino Core contain the esp_schedule change. This was required to support concurrency of some WiFi APIs with CoopTask. A #define that existed until now to allow CoopTask to work anyway, using a PR'ed feature branch of ESP8266 Arduino Core, can now be discarded, this release contains that non-backward compatible change only.